A shoulder adjuster includes a guide rail fitted so that the longitudinal direction is set as a vertical direction to a vehicle and provided with a plurality of notches formed at intervals in the longitudinal direction; and a slide member slidable in the longitudinal direction of the guide rail and having engaging parts engaging with the notches and a shoulder anchor fitting part. The notches have cut-in edge parts positioned on the underside in the vertical direction and substantially orthogonal to the longitudinal direction of the guide rail and cut-in edge parts positioned on the upside in the vertical direction and obliquely to the longitudinal direction as to expand apertures of the notches. The slide member has an anchor fitting part to provide advancement and retreatment in the engaging direction and includes an elastic member for biasing the anchor fitting part in the engaging direction. The engaging parts are integral with the anchor fitting part and biased by the elastic member in the engaging direction.