PURPOSE: To stabilize the working of an electronic device under a high level of noise, by using a photocoupler to transfer data and transmit a control signal between a pair of circuits having different levels of power supply voltage and the signal within the same housing and therefore by assuring the insulation between those two circuits.

CONSTITUTION: A housing 1 stores a microcomputer μCOP2 and an input/output device I/O3. Both the data and control signal are transferred between the μCOP2 and the I/O3 by means of photocouplers 41W45. The input given from a terminal 31 is converted into a parallel-serial signal by the I/O3 and then transmitted by the coupler 41. The control signal supplied from the μCOP2 is transmitted by couplers 42W44, and the output information given from the μCOP2 is delivered to the I/O3 by the coupler 45 after parallel-serial conversion. The signal subjected to an arithmetic process through the μCOP2 is delivered through an output terminal 32 of the I/O3. As a result, the μCOP2 is insulated electrically from the I/O3 with assurance. This process prevents the disturbance to the μCOP 2 due to a high level of noise.