Abstract: As shown in FIG. 4, as coupling holding means 20 for holding coupling between a socket S10 and a plug P10, an engaging projection 22 is formed in the plug P10 and an engaging recess 21 is formed in the socket S10 so that the socket S10 is coupled with the plug P10 accurately by an operation to engage the projection 22 with the recess 21 after an overriding operation of the projection 22. Additionally, on the plug P10 is provided a cam portion 31 which inclines in substantially a circumferential direction about a central axis running in the coupling direction and a cam follower portion 32 is formed in the socket S10. In a case where an abnormal rotational force exceeding a normal range of use is applied, an automatic release of coupling is made between the cam portion and the cam follower portion.