An optical element is fixed to a flat fixing surface of a mount. A recess is formed in the fixing surface or a lower surface of the optical element to be brought into contact with the fixing surface and solid-state cementing material is placed in the recess. The optical element is disposed so that its lower surface is in contact with the fixing surface, and the cementing material is once fused and solidified so that the optical element is fixed to the mount by way of the cementing material with the lower surface kept in contact with the fixing surface of the mount.