To provide a stroke sensor capable of linearly detecting the moving position of a movable body and being easily produced at low cost by molding, and having a structure with high strength, and to provide an actuator and a vehicle head light device.

A sensor magnet MG has an opposing face formed on a circular face S1 in a circular shape with a constant curvature radius in the longitudinal direction to face a detection face 52a of a hall IC 52. The circular face S1 has a center point O of the curvature radius r of the circular face S1 on a line L perpendicularly passing through a middle point P0 of the sensor magnet MG in the longitudinal direction on the opposite side to the hall IC 52. The hall IC 52 detects the magnetic flux density of a component perpendicular to the detection face 52a, but does not detect the magnetic flux parallel to the detection face 52a.