A locking device for seatbelt systems including a rotatable locking base with a wavy surface, a locking member provided adjacent the rotatable locking base and which is movable such that it engages with the locking base, a seatbelt portion which extends between the rotatable locking base and the locking member having an arm member with a wavy surface which senses the tension on such seatbelt portion and which engages with the locking member such that the locking member is caused to move when the arm member senses a tension on said seatbelt portion which is greater than a predetermined tension and such that the locking member presses the seatbelt portion against the rotatable base across its entire width when the locking member is caused to move by the arm member.