An image reading device includes: a platen on which a document is placed; a platen cover openable and closable with respect to the platen; a cover-open sensor which detects a fact that the open/close angle of the platen cover has passed a predetermined angle from a close state; a CCD line sensor including charge coupled devices (CCD) arranged for recording a variation in distribution for a fixed period of time of incident light when detecting passing of the platen cover; a variation amount calculator which calculates a variation amount of incident light at each distribution point on the basis of a variation in distribution recorded by the CCD line sensor; and a variation amount determination unit which determines whether or not a variation amount calculated by the variation amount calculator exceeds a predetermined threshold.