Image reading device and image forming apparatus

An image reading device comprising:

• a contact glass having a document placing surface on which a document is placed;
• an index part disposed along sides extending in a main scanning direction and in a sub-scanning direction of the contact glass, so as to be a mark for aligning a corner of the document to a reference point set at a predetermined position inside the document placing surface, the position not lying on an edge of the document placing surface;
• a reading portion disposed on an opposite side to the document placing surface so as to read a surface of the document placed on the document placing surface;
• and
• a light emitting portion held on the opposite side to the document placing surface in a movable manner in the sub-scanning direction so as to emit linear light extending in the main scanning direction toward the contact glass, wherein
• before the reading portion reads the document, the light emitting portion moves in the sub-scanning direction so that a region of the contact glass irradiated with the linear light emitted from the light emitting portion coincides with the reference point, and emits the linear light toward the contact glass.