A connector is provided with terminals (11) to be connected with terminals (32) of a mating connector (30) by bolting and a nut accommodating portion (20) for accommodating nuts (17) for bolting. An entrance (20A) enabling the insertion and withdrawal of the nuts (17) is formed at a connection surface side of the nut accommodating portion (20) with the mating connector. A nut cover (40) for preventing the nuts (17) from coming out through the entrance (20A) is detachably attachable to the nut accommodating portion (20) and includes inverted insertion preventing portions (46) for permitting a connection with the mating connector (30) when the mating connector (30) is in a proper orientation while coming into contact with the mating connector (30) to prevent the connection when the mating connector (30) is not in the proper orientation.