A dummy plug (50) includes a sealing portion (51) to be fit into a seal hole (21) in a liquid-tight manner when the dummy plug (50) is mounted into a connector (A). A support (53) is arranged behind the sealing portion (51). Two resilient locking pieces (56) link the rear end of the sealing portion (51) and the front end of the support (53) and engage with retaining portions (34) of a holder (30) when the dummy plug (50) is mounted into the connector (A). A single deformation space (57) is provided between the resilient locking pieces (56) and is shared to permit resilient deformations of the resilient locking pieces (56).