To detect the signal state of a variety of signal sources such as a switch element whose voltage/contact/current direction are determined without regard to switch operation and connection polarity.

When a signal source connected to an input terminal 1, 1' is a voltage output type, positive and negative voltage detection circuits 10 and 11 respond, thus detecting the ON/OFF of a signal source. Also, when a contact is connected, both positive and negative current detection circuits 12 and 13 respond, thus detecting ON/OFF. Further, when a switch element with polarity is connected, a corresponding current detection circuit side responds, thus detecting ON/OFF. Also, a pulse generation circuit 17 is provided, a clock pulse is selectively supplied to drive circuits 16, 16' according to voltage detection/positive current detection/negative current detection timing signals, and a signal state outputted at each timing by a signal detection circuit 18 is detected, thus detecting the ON/OFF state without considering the type of a signal source connected to the input terminal 1, 1'.