Image reading device performing light quantity detection and correction with timing corresponding to selected copying mode

In an image reading device, a document is read while being illuminated by a fluorescent lamp and then an image signal is output. A light quantity of the fluorescent lamp illuminating the document is detected with a specified timing. Then, a fluctuation in light quantity of the fluorescent lamp is corrected based on the detected light quantity. The above timing of light quantity detection is changed according to a light quantity variation characteristic, a duration of an illuminating time of the fluorescent lamp, the number of document read operations, or a set copying mode.