PURPOSE: To uniformize irradiation energy and to improve an energy using efficiency by forming the entire central part of a reflecting mirror facing a light source in a flat shape.

CONSTITUTION: The flash fixing unit is provided with a flash tube 10 as a light source, the reflecting mirror 1 disposed in the entire back of the flash tube 10, and a light transmitting glass plate 13 provided on the light emitting side of the flash tube 10. The reflecting mirror 1 is formed with an aluminum plate, and its central part is entirely flat. Further, a part along with the flash tube 10 is provided with an inverse chevron shaped projecting part 1A which smoothly and protrusively comes into contact with the flash tube 10. Thus, a wide range of the fixing side of paper can be uniformily irradiated with light energy which is almost comple-pletely uniformized in intensity, so that an energy using efficiency is enhanced effectively.