A release auxiliary device (6) includes a cover part (51) that is provided movably with respect to a receptacle (11) along the insertion/removal direction of a plug (12), and an inclined pressing part (52) that is provided in the cover part (51) and is touchable, from a lower side, to an end portion (41a) of a lever (41). The inclined pressing part (52) presses against the end portion (41a) of the lever (41) and moves the lever by moving the cover part (51) in a direction in which the plug (12) is detached, enabling release of engagement between engaging portions (42, 28).