When a tongue having a locking hole is inserted into a buckle, the edge of the locking hole is caught and locked by a catch of a rockable latch plate and is unlocked thereby when a slide button of the buckle is manually pressed in sliding movement, in which an inclined surface of the button actuates by wedge action lever-like arms of an actuating plate which, in turn, force a rockable link plate to rotate the latch plate until its catch releases the tongue. A number of springs are provided for return movements including a spring cocked by the inserted tongue for forcibly ejecting the tongue when it is released.