To take full advantage of a steering function of an electric motor, such as a steering assist function, by reducing frequency of use of temporary estimated temperature data.

A microcomputer stores the motor's estimated temperature data when an assist control is complete in an EEPROM, and uses the data as a default of operation of motor estimated temperature by reading the data when the next assist control is started. The microcomputer writes the temporary estimated temperature data in the EEPROM during the assist control to be prepared for reset due to reduction in supply voltage. The temporary estimated temperature data are not written in the EEPROM in such a situation that the electric motor is not high in temperature since the temporary estimated temperature data are supposed to be written when a current Ix flowing through the electric motor becomes larger than a set current value Ith. Accordingly, it is possible to take full advantage of the steering function of the electric motor while maintaining an overheat prevention function.