To provide a discrimination circuit capable of discriminating a plurality of types of versions of circuit boards without increasing the number of ports of a gate array.

An engine circuit board 96, and a high voltage power supply board 95 are connected at a connection point 204. In the circuit 96, a branch part 204 is composed of resistances R1 and R2 branched from the connection point 204, and in the board 95, a branch part 206 is composed of a resistance R3 branched from the connection 204. Each resistance value is changed according to the version of each substrate. The circuit 96 is provided with a PWM circuit 202, an integrating circuit 203, a comparator 201 and a gate array 200. The potential differences of the connection 204 due to the configuration difference between the branch part 204 and the branch part 206 are measured while changing a reference voltage value from 0 to 5 V stepwise by the circuit 202, and the version of each circuit board is thereby discriminated.