An integral element corresponding to a converter continues an integral operation even in a period in which a voltage converting operation of the converter is suspended. The integral output integrated in this period is invalid data. Therefore, if the invalid integral output is provided from the integral element at a time at which the voltage converting operation of the converter is resumed, it follows that the converter would be controlled by an irrelevant control value. Therefore, as described above, the voltage converting operation control unit applies a reset signal to the integral element to zero-clear the stored integral output.