1. A bearing diagnosing device for diagnosing degradation of a bearing rotatably supporting a spindle of a machine tool, comprising:a rotation speed obtainer configured to obtain a rotation speed of the spindle;
a temperature obtainer configured to obtain a temperature of the bearing;
a spindle operation storage controller configured to record a rotation time for which the spindle has rotated, at each of a plurality of classes that are defined in advance by combination of rotation speed range and temperature range, into a storage unit, based on the rotation speed and the temperature; and
a bearing degradation calculator configured to calculate bearing degradation information representing a degree of degradation of the bearing, based on the rotation time at each of the classes, stored in the storage unit,
wherein the bearing degradation calculator is configured to correct the rotation time at each class by multiplying the rotation time for each class by a predetermined coefficient for the class to calculate a corrected rotation time, which becomes relatively longer in a class with a higher rotation speed and in a class with a higher temperature and calculate the bearing degradation information by summing up the corrected rotation times for all the classes.