PURPOSE: To eliminate the need of extending a memory area for a program whose residence is not required by loading a menu program by an overlay method in a store area of a Basic Input Output System which is made resident originally and using it.

CONSTITUTION: ROMs 203, 301, and a RAM 302 constitute a first memory and a second memory, respectively, and all programs in the ROM 203, and a menu program in the ROM 301 constitute a first program and a second program, respectively. In such a state, after a first program is written in a second memory 302 of a high speed, it is made resident by inhibiting write fundamentally and the processing is executed, and also, at the time of necessity, the writing inhibition is released and a second program is overwritten in a first program. Accordingly, a necessary part of a first program can be used by a second program. In such a way, it becomes unnecessary to extend a memory area for a second program being non-resident originally of the capacity of a second memory 302.