shaped base body 51 to facilitate the insertion of the plug 50 into the aperture 44 of the socket 40 by holding the plug by the user 39 s fingers after the socket 40 is engaged with the base 20 as shown in fig5 each of the above mentioned engaging pieces 53 is downwardly bent from the plate shaped base body 51 so as to project downwardly so that it may be relisiently displaced upwardly and downwardly each of the engaging pieces 53 is adapted to be fitted in the first second and third grooves 29 30 and 31 and the free end thereof 53a is adapted to be engaged with the stopper surface 32c of the first engaging portion 32 and the inclined stopper surface 33b of the second engaging portion 33 when the plug 50 is mounted on the base 20 the engaging pieces 53 can be slidably moved along the inclined guide surfaces 32a of the first engaging portions 32 so that the plug can be elastically deformed and mounted on the base 20 thus when the engaging projection 43 of the socket 40 is fitted in the hole 25 of the base 20 the lower surface 21b of the base plate 21 is registered with the upper surface 41a of the plate shaped body 41 and also the aperture 44 is projected upwardly from the upper surface 21a of the base plate 21 refer to fig3 when the plug.