A bearing holding structure has a substantially cylindrical sleeve and a plate-shaped pressing plate. A tubular portion of the sleeve includes a housing press-fit portion having an outer peripheral surface to be press-fit into a recess of an upper housing, and a bearing press-fit portion having an inner peripheral surface to be press-fit thereinto by a bearing. A pullout prevention portion that protrudes inward to prevent pullout of the bearing is formed at one end of the tubular portion, and the pressing plate is press-fit into the other end thereof.