PURPOSE: To reduce curvature of field on a memory type fluorescent sheet and to read image information accurately by using a cylindrical mirror instead of a cylindrical lens.

CONSTITUTION: A light beam 2 which is reflected and deflecsted by a rotary polygon mirror 5 as a deflector is made incident on the cylindrical mirror 7 through an fθ lens 6. This mirror 7 reflects the light beam 2 passed through the lens 6 upon the memory type fluorescent material sheet 8 and converges it of the sheet only in a subscanning direction. The sum of the distance (a) from the lens 6 to the mirror 7 and the distance (b) from the mirror 7 to the sheet 8 is equal to the focal length f6 of the lens 6. Therefore, the light beam 2 is converged on the sheet 8 regardless of whether the polygon mirror 5 has surfaces slanted or not. Consequently, the curvature of field on the sheet 8 is reduced and image information is read accurately.