A lens holder, which supports a light receiving lens, is screwed into a barrel portion of a body holder by respectively engaging hooks, which are provided in a screwing piece, with engaging-grooves formed in an end surface of the lens holder and then rotating the screwing piece. The screwing position of the lens holder is controlled so that the rotational direction position of a set of the engaging grooves is coincided with a reference position of an adjusting piece. Subsequently, the adjusting piece is engaged with the lens holder at the reference position. Then, a focus adjustment is performed by rotating the lens holder in a positive rotation direction.