A CPU in a one-chip microcomputer executes a control operation in accordance with program data stored in part of a flash memory capable of erasing/writing data in each memory block defined in the flash memory. A write controller writes data in a specific block in the flash memory. A plurality of adjustment values for correcting variations between products which the system is to be used are written in a specific block of the flash memory after the adjustment values are determined. A control program for rewriting data stored in the flash memory is written before the one-chip microcomputer is incorporated in the system. After the one-chip microcomputer is incorporated in the system, data is written in the flash memory through the write controller. In addition, the circuit configuration of the system is stored in part of the flash memory. When a program stored in the flash memory of the system is upgraded, an upgrading program is selected on the basis of the circuit configuration is written in the flash memory by the write controller.