In a one-chip microcomputer, an electrically programmable read only memory (EPROM) is formed together with a read-only memory (ROM) and random access memory (RAM) on one semiconductor substrate. Data such as fixed data necessary in the microcomputer can be changed by the use of the EPROM. In case data are to be written in the EPROM, an EPROM writer is used. This EPROM writer outputs write data to the EPROM and checks (or verifies) the data written in the EPROM immediately thereafter. If any error is detected, a subsequent data write is interrupted. In order to inhibit the unnecessary operation interruption in case the address designated by the EPROM writer comes out of the range of the EPROM, a checking (or verifying) data signal to be fed from the one-chip microcomputer to the EPROM writer is forcibly set at a level which indicates satisfactory operation of the EPROM.