PURPOSE: To update a necessary module only without updating newly the whole program, to reduce the man-hour and to improve the maintenance, when a ROM program is updated.

CONSTITUTION: When the updating of a sub-routine D is necessary, an updating sub-routine D' to update the sub-routine D is stored to a patch area 22 of a RAM 20, and the address is written to a patch area jump table 23. When the sub-routine D is called, the updating sub-routine D' of the patch area 22 stored in an address D' written into the patch area jump table 23 at the head is called, and the updating sub-routine D' is performed in place of the sub-routine D.