An image reading apparatus comprises a correcting mechanism for correcting an output signal from a reading unit in accordance with a reference level. The correcting mechanism includes a reference element for setting the reference level and a drive mechanism for rocking the reference element between a first position in which the reference element is advanced to a reading position and a second position in which the reference element is retreated from the reading position. The drive mechanism includes a rotating member that is continuously rotated in one direction. As the rotating member rotates, the reference element rocks between the first and second positions, thereby executing correction of the output signal before image information recorded on a recording medium is read.