To increases the kind of a plug which can be connected by preventing the outer hull of the plug from interfering.

Recesses 25 and 25 which are substantially circular in plan view in the front surface of instrument body 1 are disposed in a line and plugged parts 26 and 26 into which plugs are plugged are provided respectively in the bottom surfaces of recesses 25 and 25. The plugged part 26 has plug blade insertion slots 27 and 27 for a voltage pole and a plug blade insertion slot 28 for an earth pole. Each plug blade insertion slot 27 is formed in the shape communicating a flat-type plug blade insertion slot 27a into which a flat-type plug blade is inserted, with a round pin-type plug blade insertion slot 27b into which a round pin-type plug blade is inserted. The straight lines which link the centers in the flat-type plug blade insertion slot 27a and 27a of the plug blade insertion slot 27 and 27, with the center of the round pin-type plug blade insertion slot 27b respectively are formed so as not to be parallel to each other. Therefore, by shifting the central position between the flat-type plug blade insertion slots 27a and 27b and the plug blade insertion slot 28 for the grounding pole, from the central position of the recess 25, the plug can be plugged into the recess 25 even if the dimension of the outer hell is large, and the kind of the plug which can be connected can be increased.