To fix toner on a paper for printing by properly heating it by the irradiation of a light beam.

A laser light source 20, a transmission optical element 21 and a reflection optical element 22 are arranged in turn. The element 21 is provided with negative power in a direction in parallel with the surface direction of the paper for printing 8 and orthgonally crossed to the carrying direction thereof. The reflection surface 26 of the element 22 is parabolically curved in the direction in parallel with the surf ace direction of the paper 8 and orthogonally crossed to the carrying direction thereof. Besides, it is elliptically curved in the direction in parallel with the carrying direction of the paper 8. Since the light beam emitted from the light source 20 can be linearly condensed and used for irradiating the whole width of the paper 8, the whole area of the paper 8 is heated and the toner is properly fixed.