a first connector with a first housing having opposite front and rear ends, an erroneous connection preventing rib and fitting ribs provided on an outer surface of the first housing, the fitting ribs having front ends, the erroneous connection preventing rib having a front end that is farther from the front end of the first housing than the front ends of the fitting ribs; and

a second connector with a receptacle configured to receive the first housing, an erroneous connection preventing groove and fitting grooves provided on an inner surface of the receptacle, the erroneous connection preventing groove being disposed to receive the erroneous connection preventing rib during proper connection of the first and second connectors and to collide with the receptacle during an erroneous connection, the fitting grooves being disposed to receive the fitting ribs both during the proper connection and during the erroneous connection, and to receive the fitting ribs before the collision of the erroneous connection preventing rib with the receptacle during the erroneous connection, wherein insertion of the fitting ribs in the fitting grooves achieves proper alignment of the first housing with the receptacle and ensures that the erroneous connection preventing rib will collide with a front end of the receptacle during an erroneous connection attempt.