Abstract: A cover is formed in a size which completely envelops one connector housing in a state of waiting for engagement, and a guide groove is provided on an inner wall of the cover. A flexible arm is provided on the inner wall of the cover and has a retaining portion at a distal end thereof for temporarily retaining the one connector housing in the state of waiting for engagement inside the cover. A stopper projection is provided in the one connector housing at a position opposing the guide groove for sliding in the guide groove, and another stopper projection is provided in the one connector housing at a position opposing the retaining portion of the flexible arm, the other stopper projection being temporarily retained by the retaining portion of the flexible arm. Tapered surfaces are respectively provided on the retaining portion of the flexible arm and the other stopper projection at mutually opposing positions thereof, the tapered surfaces being arranged to retain each other.