An image reading device comprising a rotary polygon mirror (6) for scanning a light flux from a light source (1), an imaging optical system (4) for forming on a reflection plane of the mirror (6) a linear image larger than the width in a main scanning direction of the one reflection plane, and a curved-face mirror (7), wherein the light source (1), the imaging optical system (4), the rotary polygon mirror (6) and the curved-face mirror (7) are disposed in different positions in a sub-scanning direction, a light flux from the imaging optical system (4) shines obliquely onto the polygon mirror (6), and a light flux reflected off the mirror (6) beams obliquely onto the curved-face mirror (7). Since one curved-face mirror can beam a light flux reflected from the rotary polygon mirror onto a surface to be scanned and the rotary polygon mirror having a small inscribed radius and many reflection planes can be used, satisfactory optical performance and high-speed feature can be realized.