To reduce cost by arranging N pieces of contact holes so as to be fittable to either of plugs having the pin terminal number not more than N, and providing front covers being installed on a front face of a housing and blindfolding a part other than the contact holes corresponding to pin terminals.

Eight contact holes 41 to 48 are arranged in a socket body 1 so as to be capable of receiving either of three pin terminals of a 3-P plug, five pin terminals of a 5-P plug and seven pin terminals of a 7-P plug. Front covers 6, 7, 8 are respectively prepared for the 3-P plug, the 5-P plug and the 7-P plug. For example, when installing the front cover 6 on a front face of a housing 2 by inserting a key 6a of the front cover 6 into a key guide hole 5, contact holes 41, 44, 45 become openings via through holes 61 to 63 of the front cover 6, and a socket for the 3-P plug blindfolded by the front cover 6 is formed in the other contact hole.