An optical scanning device includes a reflected-light passing unit having a passing region through which a portion of reflected light that is the scanning light reflected by the medium passes. An outer peripheral contour line of the passing region includes a contour curve configured with a set of points where coordinates in a direction orthogonal to a scanning direction are uniquely determined for the coordinates in the scanning direction. The contour curve renders a curve protruded toward the passing region. When a region in contact with the contour curve in the passing region is divided into a plurality of quadrilateral minute regions having equivalent areas and extending from the contour curve to a predetermined coordinate position in the scanning direction and are continuously arranged in the orthogonal direction, widths of the minute regions in the scanning direction are different for each location in the orthogonal direction.