A connector assembly has first and second housings (10, 300). Ribs (305, 306) project from an inner surface of a receptacle (301) of the second housing (300). The first housing (10) has a main body (11) that can fit into the receptacle (301), a seal ring (50) is mounted externally on the housing main body (11) and a retaining member is mounted into the main body (11). The retaining member includes a pressing surface (67, 90) for preventing detachment of the seal ring (50) and grooves (68, 86) that receive the ribs (305, 306) when the housings are connected. Reverse tapered surfaces (74, 91) are formed on the back surfaces of the grooves (68, 86) at a side facing and opposite to the pressing surface (67, 90) of the retaining member and incline toward the housing main body (11) to approach the pressing surface (67, 90).