A first receiving recess (44d) and a second receiving recess (44e) are formed on a surface of the housing, on which a pair of image forming lenses (47) are placed, to receive a first temperature sensor (101a) and a second temperature sensor (101b), and are formed in positions, in which thermal deformation characteristics of the housing are approximately identical at one side and the other side of a first straight line K1, while interposing the first straight line K1 therebetween.