A diffractive-refractive achromatic lens includes a refractive lens system exhibiting longitudinal chromatic aberration that is substantially proportional to wavelength such that the back focus of the refractive lens system decreases as the wavelength becomes shorter and a diffractive grating for correcting the longitudinal chromatic aberration of the refractive lens system. The refractive lens having such a chromatic aberration includes a positive lens having relatively small dispersion and a negative lens having relatively large dispersion. Further, the following condition (1) should be satisfied: 0.005