PURPOSE: To rewrite data on the storage device without using any special device such as an ultraviolet-ray emission device and a ROM writer by connecting the storage device to a storage device control means when a revision command signal is detected.

CONSTITUTION: When a command analytic part 7 detects the revision command signal outputted by an external computer 10, a switch 9 is switched so as to connect an interface control part 5 and a memory control part 6 to each other and a switch 8 is switched to connect a program memory 3 and the memory control part 6 mutually. Then the memory control part 6 erases data stored in the program memory 3 and also writes data for revision supplied from the external computer 10 in the program memory 3. Consequently, the program can easily be revised without damaging components at the periphery of the program memory 3.