PURPOSE: To operate a contact switch irrespective of how to insert an electric power source plug, by connecting a commercial electric power source to an input side terminal of a constant-voltage regulated power supply circuit through a resistance and a diode bridge.

CONSTITUTION: Resistances 3, 4 are connected to two input terminals of a constant- voltage regulated power supply circuit of a contact detection device 1, which are connected to an electric power source plug 6 through a diode bridge 5. In this case, if the sum of voltage drop in the resistance 3, and voltage of the constant-voltage regulated power supply circuit 2 is equal to a value of voltage drop in the resistance 4, when voltage of a commercial electric power source, etc. is made V, the earth potential of the contact detection device 1 becomes 0.5V irrespective of how to insert the electric power source plug 6. Accordingly, irrespective or how to insert the electric power source plug 6, a leakage current flows through the human body and operates. In this regard, this constitution is widely applied to not only a contact switch but also a contactless switch, etc.