An apparatus for adjusting the position of a seatbelt webbing fastened to the body of an occupant at which it is retained by the body of a vehicle has a base member secured to the vehicle body, a main shaft rotatably supported by the base member and having an external thread formed on the outer periphery thereof, and a subsidiary shaft secured to the base member and extending parallel to the main shaft. A movable member is thread-engaged with the external thread of the main shaft and slidably fitted on the subsidiary shaft, whereby, as the main shaft rotates, the movable member is prevented from rotating by the connection with the subsidiary shaft but moves along the main and subsidiary shafts through the thread engagement with the main shaft. A guide member which retains the webbing is secured to the movable member. Thus, the position of the webbing at which it is retained by the vehicle body is adjusted.