An image reading apparatus includes a drive source, a speed change mechanism provided in a transmission unit that transmits a driving force from the drive source to a scanning drive unit. The speed change mechanism is configured to change a transmission ratio of the transmission unit by selectively bringing one of first and second couplings into a connected state. The image reading apparatus further includes a selector configured to selectively bring one of the first and second couplings into the connected state, a detector configured to detect the connected state of the second coupling, and a controller configured to control a scanning position of the reading unit based on the transmission ratio changed via the second coupling after the detector detects the connected state of the second coupling after the selector changes selection from the first coupling to the second coupling.