A slider (50) is mounted on a housing (20) movable between a wait position and a detection position. The slider (50), at the detection position, is at a position opposite to a flexing direction of a locking arm (26) to prevent the locking arm (26) from flexing in an unlocking direction. In a process of shifting the slider (50) from the wait position to the detection position, a guide portion (45) gradually displaces the slider (50) toward the detection position in a direction in which the degree of locking between the slider (50) and the locking arm (26) becomes higher. At the detection position, a detection piece (54) of the slider (50) takes a downwardly inclining posture at its front portion to press down a front portion of the locking arm (26) from above.