The buckle comprises a housing in which there is disposed a support for fixing to a retaining device of the safety belt and a device for locking a bolt member secured to a portion of the safety belt. The locking device is mounted on the support to pivot about a pivot axis and comprises at least one first abutment surface against which the bolt member is retained in the locked position. The first abutment surface is disposed in such manner in the locking device that a force exerted by the belt on the bolt member produces about the pivot axis a torque for releasing the bolt member. The locking device also comprises at least one second abutment surface, a stop member movably mounted in the support and a control member for moving and withdrawing the stop member. The second abutment surface is capable, under the action of said torque, of bearing against the stop member until it is withdrawn by the control member to release the locking device. The direction of movement of the stop member is substantially parallel to the second abutment surface.