Patent ID: 8155562

Claim:
An image reading device comprising: a light source portion that irradiates light onto an original; a mirror group comprising a plurality of mirrors that forms a light path by reflecting luminous flux from the original once on each of the plurality of mirrors, and is configured so that an imaging-side mirror, which is disposed most to an imaging side on the light path, has a shortest length in a main scanning direction; an imaging lens that forms an image from luminous flux reflected by the mirror group; a reading portion that reads an image on the original based on image formation by the imaging lens, disposed in an imaging position of the imaging lens; and a casing body that houses the light source portion, the mirror group, the imaging lens, and the reading portion, to be movable in a sub-scanning direction that is orthogonal to the main scanning direction; wherein a thickness of the imaging-side mirror is formed so as to be greater than a thickness of an original-side mirror, included in the mirror group, which is disposed most to a side of the original on the light path.