PURPOSE: To change a program many times without providing an ultraviolet erase window by updating the value stored in a write counter before the start of data write to a PROM (programmable ROM) selected by a PROM selector.

CONSTITUTION: First to third PROMs 1-4 to 1-6, PROM selectors 1-7 and 1-8, a write circuit 1-10, a read circuit 1-11, and a central processing unit 1-12 are provided as shown in the figure. That is, n PROMs which are applied to a PROM incorporating microcomputer and in which a program having a maximum of m steps is stored, a write counter 1-1 consisting of the PROM which counts the frequency in write to these PROMs, and a means which updates values stored in the PROM selector, which selects one of n PROMs in accordance with the value stored in the write counter 1-1, and the write counter 1-1 are provided. Thus, the program can be changed a maximum of n times without providing an ultraviolet erase window.