A control program is previously stored in a flash ROM, and when update of the control program stored in the flash ROM is requested, control is changed to that based on a program stored in a mask ROM, and a control program as the subject of update is received and written into the flash ROM under the control of the program stored in the mask ROM. When writing of the new control program into the flash ROM has ended and the update of the control program in the flash ROM has been completed, the control is changed to that based on the updated control program. Further, information indicating whether or not the update of the control program has been normally completed is stored in an EEPROM, and control processing is performed based on the control program stored in the flash ROM or the control program stored in the mask ROM in accordance with the information.