PURPOSE: To proceed to a new firmware whose contents are changed even during an on-line operation relating to a firmware changing system during the on-line operation in a processor to which a ROM loaded with the firmware is detachably connected.

CONSTITUTION: The firmware is constituted of the plural blocks of a function unit and a jump table for making access to the respective function blocks and a processing part 1 operated by the firmware of a RAM 4 written from the ROM to the RAM 4 at the time of starting the operation is provided. At the time of changing the firmware, the new ROM 2 constituted of patch data including the patch entry address of the function unit and the changed firmware is prepared and when it is switched with the original ROM, an informing signal is generated from a ROM detachment detection means 3 and the processing part 1 writes the patch data to the RAM 4 by the informing signal and simultaneously writes the entry address to the jump table 4a.