PURPOSE: To easily change a program in the state of mounting onto a printed circuit board by directly applying the data of serial bits, which is applied from the outside, to a read only memory and reloading the data.

CONSTITUTION: An electrically erasable and reloadable read only memory (EEPROM) 20 and an input/output port 3 are connected through a bus 5 to a CPU 1, and a write control circuit 4 is connected to the EEPROM 20. Further, a serial data communication circuit 6 and an address counter 7 are provided. The serial data communication circuit 6 receives the serial data applied from the outside, converts it to the data of parallel bits and applies the converted data to the EEPROM 20, and the data is written in the address of the EEPROM 20, which is designated by the address counter 7, by the control circuit 4. Thus, in the state of mounting the microcomputer integrated circuit device onto the printed circuit board, the new program data or the like can be directly written from the outside without exerting any influence upon the external circuit connected to the other input/output terminal.