A belt fixing unit, an image forming apparatus, and a toner material are disclosed capable of obviating undesirable fixing belt deformation and low image quality. The coefficient is dependent on the specific design of the cryogen valve and nozzle, and values up to 11 500 Wm2K values were measured for a 100 ms long spurts. And, the objective lens actuator not only controls but also drives the optical pickup head. A center of curvature of the curved mirror is placed substantially optically midway between the exit pupil and the left optical system the exit pupil of the right optical system. In a cross section perpendicular to the light incident surface and passing through the first curved surface and the second curved surface, the first curved surface and the cross section intersect in a first curved line, and the second curved surface and the cross section intersect in a second curved line. This triangular shape allows the clubs to have higher rotational moments of inertia in both the vertical and horizontal directions, and a lower center of gravity.