PURPOSE: To maintain coupling even when the force tending to release the coupling is applied by pinching a coupling section coupled with a coupled section on the side face of the facing surface of a main body case combined with halved cases with one of the halved cases formed with a plug body and the coupled section.

CONSTITUTION: A coupling section 26a and a coupled section 24a formed on halved cases 13, 14 of a flat main body 11 respectively and facing each other are coupled, and the halved cases 13, 14 are combined and fixed. A plug body 43 fixed with a pair of plug blades 44 in parallel is slidably stored in the inside, and the plug blades 44 can be kept in the advanced state or retreated state from the plug advance section 19 of the main body case 11. The coupling section 26a formed on the plug advance section 19 and coupled with the coupled section 24a is pinched by one halved case 13 formed with the coupled section 24a and the plug body 43 protrusively held with the plug blades 44.