PURPOSE: To make sufficient safety securable by installing a device which discriminates the polarity of connection to the live line and neutral line of a power source and also another device for informing of this discriminating device.

CONSTITUTION: A commercial AC power source 1 consists of a live line L and a neutral line N, and a live side terminal 2L of a plug socket 2 is connected to the live line L, and a neutral side terminal 2N of the plug socket 2 to the neutral line N in wiring, respectively. There are provided with a device 30 for discriminating the polarity of connection to the live line L and neutral line N of the power source 1, and another device 40 for forming of this discriminated result. And, if the connecting polarities are not accorded with each other, the effect is informed. With this constitution, whether the polarity of connection to the live line L and neutral line N of the power source is accorded with each other or not is made recognizable to a user easily and accurately so that sufficient safety is securable.