To prevent a plug connected to a socket from slipping out of the socket by an unexpected accident.

A second engaging recessed part 2g is formed on an outer peripheral surface of a socket body 2. A cover 8 is suitable for the outer periphery of the socket body 2 so as to be movable between a movement allowable position and a movement checking position. A right end surface of the cover 9 substantially contacts with a step height surface 7d of an operation member 7 positioned in an engaging position when the cover 9 is positioned in the movement checking position. An engaging projection part 9a is formed on an inner peripheral surface of the cover 9. The engaging projection part 9a is engageably-disengeably fitted in the second engaging recessed part 2g when the cover 9 is positioned in the movement checking position. The engaging projection part 9a is formed so as to be capable of escaping from the second engaging recessed part 2g when predetermined force larger than energizing force of a second energizing member 8 acts leftward on the cover 9.