PURPOSE: To reduce the number of wirings between a digital control part and a D/A conversion part.

CONSTITUTION: A digital control part 1 is provided with a serial conversion part 14. A D/A conversion part 2 is also provided with a serial conversion part 24. The digital control part 1 and the D/A conversion part 2 are wired with each other by means of a data line 5, a clock line 6 and a write line 7 via an isolation circuit 4, respectively. When a digital value is calculated by a microprocessor 11, data are fed to a shift register 15 of the serial conversion part 14 and transmitted to the data line 5 bit by bit. When the prescribed number of bit data has been transmitted, a writing signal indicating the start of D/A conversion is outputted to the write line 7. On the other hand, the D/A conversion part 2 takes in the contents of the data to a shift register 25 and transmits the entire contents to a D/A conversion circuit 23 with the writing signal.