A release auxiliary device (6) includes a base member (51) fixedly provided with respect to a receptacle (11), and an auxiliary lever (52) provided in an elastically deformable manner with respect to the base member (51). The auxiliary lever (52) has a base part (61) extending to the base member (51), a pressing part that extends to the base part and is touchable to an end portion of a lever (41), and an operating part (63) that extends to the pressing part and is disposed at a position closer to the body of the plug than the end portion of the lever is. By operating the operating part (63) with fingers, the pressing part (62) presses against the end portion of the lever (41) and moves the same, so that the engagement between engaging portions (42, 28) for retention can be released.