An optical axis of at least one surface of a resin-made diffracting lens is shifted in a main scanning direction with respect to an incident beam. A synchronous detection can cancel a problem of a misalignment in the main scanning direction due to a temperature variation. A light reflected from a second surface of the resin-made diffractive lens condenses on a position that is displaced in an optical axis direction from an optical beam outgoing point of a semiconductor laser, and thereby the light reflected again from the semiconductor laser does not form an image on a scanned surface and an impact on the image becomes low.