An image reading apparatus has a platen, first and second carriages, a light source supported on the first carriage, a first mirror to deflect reflected light from the original document toward the second carriage, one or more second mirrors supported on the second carriage to guide the light from the first mirror to image reading device, and rail members. The first carriage has first and second mirror support portions for supporting one side edge portion of the first mirror by two points, and a third mirror support portion for supporting the other side edge portion by one point. The first carriage is supported slidably at its opposite side edge portions respectively on the rail members via slide members disposed at four right and left places including two front and back places, and a height position is adjustable for supporting the first mirror by one point.