PURPOSE: To offer and A/D converting circuit whose constitution is simple and whose cost is low by controlling digitally a comparing circuit which has combined a comparator and a D/A converter, through an isolating circuit by a photocoupler.

CONSTITUTION: A signal to be measured is inputted to a comparator 5 through an amplifier 4 from a terminal A. On the other hand, a digital signal of 8 bits sent from a CPU 8 through each photocoupler 13 of an I/O port 7 and an isolating circuit 2 is converted to an analog signal by a D/A converter 6, and sent to the comparator 5. A comparing output of "0" or "1" level is inputted to the I/O port 7 through the isolating circuit 2 in accordance with the magnitude of two signals from the comparator 5. In accordance with whether this inputted comparing output is "0" level or "1" level, the CPU 8 sends out an adding signal of "+1" constituted of 8 bits or a subtracting signal of "-1" to the D/A converter 6 through the isolating circuit 2 from the I/O port 7. At the same time, it is sent to other circuit, too, through a terminal B.