PURPOSE: To reduce the power consumption by transferring the content of an ROM to an RAM when the electric power is supplied, executing an instruction in accordance with the content of the RAM, and cutting the power supply to the ROM while an early stage instruction is executed.

CONSTITUTION: When the electric power is supplied, a switch SW1 is closed and the power is conducted to all parts including an ROM3. A switch SW5 is set to a condition where writing and reading out to an RAM4 can be enabled. A CPU2 reads out all the contents of the ROM3 and writes the contents in the RAM4 through the switch 5. When the transfer is completed, the CPU2 controls the switch 1 through an I/O port 6 and cuts the power supply to the ROM3 in accordance with an early stage instruction in a program stored in the RAM4. The CPU2 simultaneously controls the switch SW5 and fixes the switch SW5 to the reading out condition. Thereafter, the CPU2 performs necessary control in accordance with the content stored in the RAM4.