a buckle comprises a plug including a protuberant arm having first engaging means ; and a socket adapted to be releasably coupled with the plug . the lower wall of the socket has a cantilevered engaging flap provided on its inner surface thereof and having second engaging means for snapping engagement with the first engaging means for coupling of the plug and the socket . the upper wall of the socket has a cantilevered resilient presser flap overhanding the cantilevered engaging flap and being yieldable downwardly to thus depress the resilient engaging flap , thereby bringing the second locking means out of engagement with the first engaging means of the protuberant arm for uncoupling the plug from the socket . the buckle further includes a locking plate slidably mounted on the presser flap for reciprocation between its locking position and its unlocking position .