To reduce the delay of a clock signal to be transmitted to each of a plurality of unit devices when those ground-separated unit devices where clock signals should be synchronously operated are cascade-connected.

This unit device is provided with a clock source connector 12 for connecting a clock source 14, a reception connector 17, a transmission connector 18, a unit functioning part 16, and a clock distribution circuit 20 connected to each of them. The clock distribution circuit 20 includes a photocoupler 15, and the photocoupler 15 is connected between the unit functioning part 16 and the reception connector 17, and not connected between the reception connector 17 and the transmission connector 18.