To eliminate the need for complicated operation by automatically selecting a desired program storage means out of plural program storage means at the time of start.

When the level of a positive/negative inverted reset signal (bar RESET signal) is low, a signal at a high level is supplied to the bar WE input of an EEPROM 12, a signal at a low level is supplied to the bar OE input and a signal at a low level is supplied to the bar CE input respectively. An address bus is turned to '0' and as a result, the data of the address '0' are outputted from the EEPROM 12 to a data bus. At the rising of the bar RESET signal, on the other hand, the least significant bit value of an output from the EEPROM 12 is held by a hold circuit 16 and supplied to selectors 17 and 18. Thus, since the selector signals of the selectors 17 and 18 are determined before the start of a microprocessor, an effective program memory is automatically selected, and a CPU is normally started.