1. A flush water tank apparatus storing flush water for flush a toilet, comprising:a flush water tank having a discharge opening;
a discharge valve mechanism having a discharge valve configured to open and close the discharge opening by an operation of the discharge valve;
a power drive unit mounted so as to be concealed in an interior of the flush water tank, and capable of electrically operating the discharge valve of the discharge valve mechanism, wherein the power drive unit comprises a motor rotationally driven by electricity and an output shaft for outputting rotation of the motor; and a housing portion formed so as to surround an outlet of a drive unit interior air passage extending from the motor of the power drive unit inside the flush water tank;
wherein the housing portion is opened toward an outside of the flush water tank, and an air passage is formed to communicate from the motor of the power drive unit through the housing portion to the exterior of the flush water tank.