PURPOSE: To easily rewrite data stored in a flash PROM so as to simplify the maintenance work of the ROM by mounting the PROM on a printer device storing CPU controlling data and rewriting the data through an I/F.

CONSTITUTION: A CPU 1 is connected to a CGROM 2, RAM 3, PROMC 4, I/F 6, SW 7, and OPP 9 through an address data bus 10. When a flash PROM rewriting button is pressed in the SW 7 or the I/F 6 receives a flash PROM rewriting control instruction to the CPU 1, the CPU 1 transfers the flash PROM rewriting control instruction from the CGROM 2 or a flash PROM 8 to the RAM 3. The CPU 1 controls the PROMC 4 in accordance with the instruction and the PROMC 4 changes its control to the control of data writing to the flash PROM 8 by switching the voltage of a VSW 5. The CPU 1 rewrites the content of the flash PROM 8 by using data from the I/F 6.