Disclosed is an image reading apparatus which includes an illumination system, a reading unit, and an imaging optical system for imaging imagewise information provided on a surface of an original as illuminated by the illumination system, upon a surface of the reading unit. The imaging optical system includes, in an order from the original surface side, a first lens group of negative refractive power, a second lens group of positive refractive power, and a third lens group of negative refractive power. The image reading apparatus has a focal point position adjusting function and a focal length changing function based on movement of at least one of the lens groups of the imaging optical system. In the imaging optical system, a condition 0.01