PURPOSE: To reduce the power consumption and to maintain the speed of an arithmetic process, by providing a power supply ON/OFF control circuit between a microprocessor and a main power source and therefore ensuring a connection of the main power source by means of a working interruption to the microprocessor and turning off the main power source with the arithmetic stop signal.

CONSTITUTION: A microprocessor MPU is formed with an N-MOS IC, and a power supply ON/OFF control circuit PCC is provided between a power supply terminal VCC and a main power source E of the MPU. A transistor TR and an FF are provided to the circuit PCC, and the FF is reset by the start signal I supplied from an input/output control circuit IOC. Thus the TR conducts to supply the voltage to a clock oscillator CLK, a program memory PM, etc. At the same time, the FF is set by the arithmetic end signal H when an operation of the MPU is over. Thus the TR is made nonconductive to cut off the power supply to the MPU and other devices. As a result, the power consumption is reduced.