An electric motor controller for a vehicle has a voltage detecting section for detecting the voltage of a filter capacitor inserted on a direct current side of an inverter device including a semiconductor element for large electric power; an electric current detecting section for detecting an output electric current of the inverter device; and a temperature detecting section arranged in a cooling means of the semiconductor element for large electric power. Loss generated by a switching operation of the semiconductor element for large electric power is sequentially calculated by outputs of the detecting sections. Excessive temperature is detected when junction temperature of the large electric power semiconductor element calculated by this loss reaches an allowable temperature.