To minimize the energizing time to a connected resistance and to reduce the heat generation of the resistance by connecting a switch circuit to an input side of a digital signal input circuit and to have it operate in time division.

When plural pieces of digital signal input circuits are arranged in parallel, switching devices 11 to 14 that open and close the circuits are connected to the primary side of each photocouplers 41 to 44 and also FF1 to FF4 are connected to a secondary side. The same control signal SW1 to SW4 are sent to each devices 11 to 14 and the FF1 to FF4. Thereby, an energizing times to inserted resistances R11 to R12 are reduced, and the generation of heat is suppressed in such a manner that each photocoupler 41 to 44 operates in time division.