An image forming lens is fixed to a protruding seat part, which protrudes from a bottom wall part of a casing, by using an adhesive. A concave portion is formed at an end surface of a protruding side of the protruding seat part to receive a temperature sensor. The adhesive is filled in the concave portion to fix the temperature sensor and is interposed between the end surface of the protruding side of the protruding seat part and the image forming lens to fix the image forming lens to the protruding seat part.