The operation of a lock member and the check thereof are easily and reliably enabled with a simple configuration while achieving downsizing. When a movable lock engagement part is moved along a lock guide part in a state in which a fixed lock latch part provided on either one of first and second electrical connectors is in an arrangement relation that the fixed lock latch part is overlapped with the lock guide part provided on the other one in a mating direction, the movable lock engagement part is engaged with or released from the fixed lock latch part. An operation of engaging or releasing the movable lock engagement part with/from the fixed lock latch part is carried out in a state in which the operation can be visually checked through a housing-operation opening and facing the electrical connectors.