A structure for applying a pre-load to a rolling bearing of a rotational mirror driving unit, the rotational mirror driving unit including the rolling bearing, a scanning mirror, and a motor, the rolling bearing having an outer ring and an inner ring to which a vertically rotating shaft is fixed as a first rotating shaft, the scanning mirror constituting an optical system and being attached to the vertically rotating shaft, and the motor rotationally driving the vertically rotating shaft, the structure comprising: a housing a sleeve that engages with the outer ring; a plate spring that detachably engages with the sleeve and that has an insertion hole, the insertion hole being provided to a fastening part that is to be fixed to the housing body being a fixing member; and a ring that detachably joins the sleeve and the plate spring, wherein the sleeve has a cylindrical part, an annular engaging part provided to an end of the cylindrical part, and a flange provided to the other end of the cylindrical part, the ring is formed with a female screw at an inner periphery, the cylindrical part is formed with a male screw at an outer periphery, the plate spring is formed with a circular hole at a center, the plate spring is fixed to the sleeve by interposing the plate spring between the flange and the ring in which the female screw at the inner periphery of the ring is screwed to the male screw at the outer periphery of the cylindrical part that is inserted into the circular hole, the housing body is formed with a screw hole, the plate spring is fixed to the housing body in an elastically deformed manner by screwing a screw to the screw hole of the housing body through the insertion hole of the plate spring, and the elasticity of the elastically deformed plate spring applies a pre-load to the outer ring via the sleeve in which the annual engaging part of the sleeve is brought into contact with the outer ring. a housing a sleeve that engages with the outer ring; a plate spring that detachably engages with the sleeve and that has an insertion hole, the insertion hole being provided to a fastening part that is to be fixed to the housing body being a fixing member; and a ring that detachably joins the sleeve and the plate spring, wherein the sleeve has a cylindrical part, an annular engaging part provided to an end of the cylindrical part, and a flange provided to the other end of the cylindrical part, the ring is formed with a female screw at an inner periphery, the cylindrical part is formed with a male screw at an outer periphery, the plate spring is formed with a circular hole at a center, the plate spring is fixed to the sleeve by interposing the plate spring between the flange and the ring in which the female screw at the inner periphery of the ring is screwed to the male screw at the outer periphery of the cylindrical part that is inserted into the circular hole, the housing body is formed with a screw hole, the plate spring is fixed to the housing body in an elastically deformed manner by screwing a screw to the screw hole of the housing body through the insertion hole of the plate spring, and the elasticity of the elastically deformed plate spring applies a pre-load to the outer ring via the sleeve in which the annual engaging part of the sleeve is brought into contact with the outer ring.