An image reading device includes a contact glass having a document placing surface, an index part to be an index for aligning a corner of a document to a reference point within the document placing surface, a reading portion configured to read the document, and a light emitting portion configured to emit linear light extending in a main scanning direction. When a predetermined point apart from the index part within the document placing surface is set as the reference point, the light emitting portion moves in a sub-scanning direction so that a region of the contact glass irradiated with the linear light coincides with the predetermined point, and emits the linear light.