A buckle (10) including a plug (30) and a socket (20) is provided with a posture-adjuster that adjusts relative postures of the plug (30) and the socket (20) in a direction that leads a longitudinal axis (A2) of the plug (30) and a longitudinal axis (A1) of the socket (20) to be parallel when the plug (30) is relatively moved in a direction toward inside of the socket (20) from an insertion slot (21) thereof. The posture-adjuster has a guide path (24) that is provided to the socket (20) and has a width that becomes wider as being closer to the front end side thereof as well as a step portion (36) as a guided member that is provided to the plug (30) and guided by the guide path (24).