A shoulder adjuster apparatus 10 includes an adjusting base 1 secured to a car body, a sliding base 2 mounted on the adjusting base 1, and a garnish 3 sliding together with the sliding base 2. The adjusting base 1 includes side walls 11 formed by bending both edges, which are arranged in the width direction, at about 90 degrees with respect to a flat surface 13 to be attached to the car body, and engaging portions 12 formed by bending the ends of the side walls 11 inwardly. The engaging portions 12 and the side walls 11 form acute angles. The sliding base 2 is slidably inserted into a sliding part, which is defined by the side walls 11, the engaging portions 12, and the flat surface 13. According to the structure as described above, a shoulder adjuster apparatus having excellent shock absorbency, capable of preventing degradation of performance due to deformation and the like, and having excellent safety characteristics can be provided.