To easily detect the occurrence of a half falling-off state, and to easily specify a falling-off place by announcing this by detecting the change when half falling-off of a connecting state is caused, and using a detecting IC and an electric switching device as a detecting means.

A mechanism containing a microswitch 12 being an electric switching device and a detecting pin 2 is used as a detecting means. A buzzer 4 is used as an announcing means. These means are housed in an electric plug case 3, and power source energy for buzzing the buzzer 4 is taken from a power source line 15 being a part of a transmitting means. When a power plug is inserted into a power electric outlet, electric energy is supplied from a power circuit 13, so that the buzzer 4 buzzes by an announciator drive circuit 14. When the plug is deeply inserted further, the detecting pin 2 is pushed in, so that the microswitch 12 acts to stop a buzz of the buzzer 4. When joining of the plug and a socket is loosened, the microswitch 12 operates and the buzzer 4 buzzes again to announce abnormality of a connecting state to a user.