A waterproof connector has a housing (2) for accommodating terminal fittings (1) connected to ends of wires (W). An engaging portion (28) is formed on the outer surface of the housing (2). A one-piece rubber plug (15) is mounted in a rear part of the housing (2) to collectively seal the wires (W). A holder (20) is fit to the rear part of the housing (2) to retain the one-piece rubber plug (15) and a cover (27) is arranged on a rear part of the holder (20). The holder (20) is formed with an opening (46) for exposing the engaging portion (28) if the holder (20) is mounted properly on the housing (2) and the cover (27) is formed with a lock (57) that can engage the exposed engaging portion (28).