PURPOSE: To rewrite a microprogram in a central processing unit during the operation of the central processing unit by providing the central processing unit containing a microinstruction dedicated for microprogram rewrite, and executing a microprogram rewrite instruction by the central processing unit.

CONSTITUTION: A main memory device 1 includes software 2 which prepares the rewrite of the microprogram based on information inputted from a device operator, a microprogram rewrite address storage area 3 generated by the software 2, and a microprogram rewrite data storage area 4. The central processing unit 6 is equipped with a main memory device access control part 7, a microprogram control part 8 equipped with a microprogram storage area 10, and an arithmetic processing control part 11. When the software 2 executes the microprogram rewrite instruction, the microprogram 12 in the central processing unit 6 is operated, which rewrite part of the program in the microprogram area 10.