1. A load measuring apparatus for a working machine applicable to a working machine having a body frame, front wheels and rear wheels rotatably disposed on the body frame, a vessel tiltably mounted on the body frame for carrying a cargo, and front suspension cylinders and rear suspension cylinders disposed between the body frame and the front wheels and rear wheels, respectively, for reducing shocks to a body during traveling, to measure a load of the cargo carried on the vessel, wherein the front suspension cylinders include a left front suspension cylinder and a right front suspension cylinder, the rear suspension cylinders include a left rear suspension cylinder and a right rear suspension cylinder, the load measuring apparatus for a working machine comprising:a front-wheel rotational speed sensor that detects rotational speed of the front wheels;

a rear-wheel rotational speed sensor that detects rotational speed of the rear wheels;

a front load acquisition unit that acquires a front load applied to the front suspension cylinders;

a rear load acquisition unit that acquires a rear load applied to the rear suspension cylinders;

a surface resistance detection unit that detects a surface resistance of a travel road for the working machine;

a total load computing unit that based on the front load acquired by the front load acquisition unit and the rear load acquired by the rear load acquisition unit, computes a total load acting on the front suspension cylinders and rear suspension cylinders to determine the load of the cargo;

a computed total load value correction unit that corrects the total load computed by the total load computing unit, according to the surface resistance detected by the surface resistance detection unit so as to obtain a corrected total cargo load value;

a storage device that stores a first correlation between a front/rear load ratio as a ratio of the front load to the rear load and a total load eccentric-loading-dependent correction ratio for correcting an error caused by eccentricity of the cargo on the vessel, a second correlation between the surface resistance and a total load correction coefficient as a correction coefficient for correcting an error caused by the surface resistance on the total load, a third correlation between the surface resistance and a front/rear load ratio correction coefficient as a correction coefficient for correcting an error caused by the surface resistance on the front/rear load ratio, and a body-side load of the working machine, a cylinder cross-sectional area which is a cross-sectional area of each of the front suspension cylinders and the rear suspension cylinders, and surface friction characteristics;

a front/rear load ratio computing unit that computes the front/rear load ratio from a ratio of the front load acquired by the front load acquisition unit to the rear load acquired by the rear load acquisition unit; and

a correction ratio computing unit that computes the total load eccentric-loading-dependent correction ratio by applying the front/rear load ratio computed by the front/rear load ratio computing unit, to the first correlation stored in the storage device,

wherein the front load acquisition unit comprises:

a first multiplier that computes a left front side load acting on the left front suspension cylinder by multiplying a suspension pressure in the left front suspension cylinder by the cylinder cross sectional area,

a second multiplier that computes a right front side load acting on the right front suspension cylinder by multiplying a suspension pressure in the right front suspension cylinder by the cylinder cross-sectional area, and

a first adder that computes the front side load by adding the left front side load and the right side front side load;

wherein the rear load acquisition unit comprises:

a third multiplier that computes a left rear side load acting on the left rear suspension cylinder by multiplying a suspension pressure in the left rear suspension cylinder by the cylinder cross sectional area,

a fourth multiplier that computes a right rear side load acting on the right rear suspension cylinder by multiplying a suspension pressure in the right rear suspension cylinder by the cylinder cross sectional area, and

a second adder that computes the rear side load by adding the left rear side load and the right side rear side load;

wherein the surface resistance detection unit

computes a slip ratio by dividing a value obtained by subtracting the rotational speed of the front wheel detected by the front-wheel rotational speed sensors from the rotational speed of the rear wheel detected by the rear-wheel rotational speed sensors, by the rotational speed of the rear wheel;

computes a road surface friction coefficient on the basis of the computed slip ratio and a road surface friction characteristics, and

obtains the surface resistance of the travel road by multiplying the computed road surface friction coefficient by any one of the left front load, the right front load, the left rear load, and the right rear load;

wherein the computed total load value correction unit includes:

a total load correction coefficient computing unit that computes the total load correction coefficient by applying the surface resistance, which has been detected by the surface resistance detection unit, to the second correlation stored in the storage device;

a total load correction coefficient multiplier that multiplies the total load correction coefficient, which has been computed by the total load correction coefficient computing unit, by the total load computed by the total load computing unit so as to obtain a surface resistance eliminated total load;

a front/rear load ratio correction coefficient computing unit that computes the front/rear load ratio correction coefficient by applying the surface resistance detected by the surface resistance detection unit, to the third correlation stored in the storage device;

a front/rear load ratio correction coefficient multiplier that multiplies the front/rear load ratio correction coefficient computed by the front/rear load ratio correction coefficient computing unit, by the total load eccentric-loading-dependent correction ratio computed by the correction ratio computing unit; so as to obtain a surface resistance eliminated eccentric-loading-dependent correction ratio;

a divider that computes an error corrected total load by dividing the surface resistance eliminated total load by the surface resistance eliminated eccentric-loading-dependent correction ratio; and

a subtractor that computes the corrected cargo load value by subtracting the vehicle body-side load from the error corrected total load.