There is provided a gradient index rod lens unit having a desired chromatic aberration. The gradient index rod lens unit is comprised of two gradient index rod lenses 11 and 12 with different chromatic aberrations and the entire lengths thereof adjusted, which are disposed in series with the optical axes thereof in alignment. The chromatic aberration of the gradient index rod lens unit can be set to a value falling within chromatic aberration ranges inherently possessed by the respective gradient index rod lenses 11 and 12.