PURPOSE: To modify a microprogram easily in a short time, by forcedly jumping an address to an RAM address for patching when the address coincide with a patching address.

CONSTITUTION: An address to be patched (patching address 1) is set in a patching address setting circuit 2-1. When the address of an address bus 100 coincides with the patching address 1, an address comparator circuit 3-1 makes an address coincident line 30-1 have a logic "1". At the same time, an ROM inhibiting line 40 which is the output of an OR circuit 4 also becomes logic "1" and an ROM1 is set to a readout-impossible condition. Then another OR circuit 9 becomes logic "1" and a transmitting circuit 5 becomes an inactivated condition. At the same time a jump address is sent out from a jump address generating circuit 6 and inputted into an RAM8. When the jump address is inputted, the microinstruction of the jump address 1 of the RAM8 is executed instead of the ROM1 and modification of a microprogram is performed easily in a short time.