A seat belt shoulder adjuster has a rail and a slider, the slider having formed in it a webbing returning part through which the seat belt is returned to the passenger compartment of a vehicle. The rail is disposed along a pillar and fixed thereto. The slider is mounted to the rail so as to enable up and down movement with respect thereto, and webbing of a seat belt passes between the rail and the slider via the webbing returning part, into the passenger compartment of the vehicle.