Disclosed herein is an anchor mounting device suitable for use in fixing an upper anchor, i.e., the deflector of a seat belt on the body of a vehicle while allowing the occupant to adjust the height of the deflector in accordance with his physical constitution. It comprises a base, a slide member guided along the base and carrying an anchor plate thereon, at least one latch piece mounted on the slide member and releasably kept in engagement with the base and means normally biasing the latch piece toward the engaged position. The slide member is provided with an interlocking portion which is capable of giving an additional support to the latch piece. It is easy to operate and, owing to the effective utilization of the base and slide member, can withstand heavy loads.