PURPOSE: To rewrite data (programs) after shipping products and to cope with problems generated after going on the market.

CONSTITUTION: An OTP (one time programmable) memory inside this microcomputer is composed of an EPROM 11 and a mask ROM 12 and a data (program) part 11a, a program part 11b and a monitor part 12a are formed in the memory space. The EPROM 11 is provided with bank memories composed of three banks 0, 1 and 2 on the same memory space in the data (program) part 11a. Thus, even after shipping the products, by rewriting the contents of a bank register constituted of one of the cells of the EPROM 11, one of the plural bank memories is successively selected, the rewrite of the contents of the EPROM 11 is made possible and appropriate measures are relatively easily taken even after the products go on the market.