A resistance calculation activator determines whether or not an electric car is stopped on the basis of a drive command signal and an external signal, and activates a resistance calculator if a powering command is input as the drive command when it is determined that the electric car is stopped. The activated resistance calculator computes, within a resistance calculation period, a resistance value of an AC motor that produces driving force for the electric car, on the basis of a d-axis voltage command value and a d-axis current value supplied to the AC motor.