An attempt to fit an improper combination of housings (10, 30) together brings a projection (32R) of the wrong housing (30) into abutting engagement with an interference portion (15) in a fitting recess (12). Abutment surfaces (15a, 32Ra) are inclined in directions to move an inner peripheral surface of the fitting recess (12) towards an outer peripheral surface of the wrong housing (30). Therefore, the fitting recess (12) cannot deform and spread out, and the fitting of the wrong housing (30) cannot proceed by force.