A connector etc. that prevents wrong insertion and does not require large rotation of plug etc., when inserting a plug to a socket, is provided. Plug-in connector 1 comprises pin plug 10 and receptacle 20, and pin plug 10 and receptacle 20 are connected. On pin cover body 14 of pin plug 10, plural pin plug key protrusions 15 are constructed separately along the circumferential direction, and on socket cover 27 of receptacle 20, plural sets 28a-28e of plural key grooves 28 separated along the circumferential direction, that plural pin plug key protrusions 15 fit into each set, are arranged with shifting along the circumferential direction. Receptacle 20 can be substituted by a socket plug 30.