PURPOSE: To obtain the slave microcomputer which effectively utilizes pins of a chip and is rich in flexibility by controlling the serial input of an external program, and storing the external program in a volatile RAM and actuating it on the RAM.

CONSTITUTION: This slave microcomputer is provided with a serial input/output control circuit 14 which controls the serial input for the external program and a data transfer control circuit 18 which holds a CPU 11 when the external program is inputted, transfers the external program to the volatile random access memory(RAM) 16, and releases the CPU 11 from being held after the transfer. This slave microcomputer 1 controls the serial input of the external program from a master microcomputer 2 by the serial input/output control circuit 14, and stores the external program in the volatile RAM 16 by the data transfer control circuit 18 and actuates it on the RAM 16. Therefore, the number of pins required for the data transfer is minimized.