An image reading device of the present invention includes a glass platen mounted on a support member. A rotary cam, a home position sensor and so forth allow the support member to move in reciprocation in a subscanning direction. When a document is conveyed along the glass platen, the glass platen is moved in the subscanning direction together with the support member. Consequently, a reading section and the reading position of the glass platen move relative to each other, obviating black stripes and other defects ascribable to contamination.