PURPOSE: To obtain a system having high reliability, while suppressing an increase in the cost, by effectively utilizing a remainder area of a memory.

CONSTITUTION: Addresses AWA+m(=B-1), BWB+m(=C) (m denotes length of a unit area) become n-th and (n+1)-th unit areas of this program memory, respectively, a program P is stored in an area of addresses AWQ, and an area of addresses Q+1WC of a second unit area becomes a remainder area. By overlapping and storing at least a part (r)W(s) of program whose length can be stored in this remainder area, a part of the program is dualized and stored. In such a way, a part of the program can be dualized and stored without adding a memory element and a system having high reliability can be obtained.