A zoom lens comprising, in order from an object side to an image side: a first lens unit having a positive refractive power and being configured to move during focusing; a second lens unit having a negative refractive power and being configured to move towards the image side during zooming from a wide-angle end to a telephoto end; a third lens unit having a positive refractive power and being configured to move during zooming to compensate for the variation of an image plane caused by zooming; and a fourth lens unit having a positive refractive power and being configured not to move for zooming, the second and third lens units (U2, U3) constituting a system; characterized in that the second lens unit includes at least one positive lens and at least one negative lens, and that average values of Abbe number (ν) and relative partial dispersion (θ) of materials of the at least one negative lens (νna, θna) and average values of Abbe number (ν) and relative partial dispersion (θ) of materials of the at least one positive lens (νpa, θpa) and a focal length of the first lens unit and a focal length of the zoom lens at a telephoto end (ftele) satisfy the following conditions: - 10.0 × 10 - 3 < θpa - θna / νpa - νna < - 3.7 × 10 - 3 3.9 < ftele / f ⁢ 1 < 6.0; wherein the value of the secondary spectrum of axial chromatic aberration in the first lens unit can be effectively decreased by the value of the secondary spectrum in the second lens unit; and the zoom lens corrects chromatic aberration that may occur in each lens to the level as low as approximately 0, so that for each lens unit, the image forming positions of C-line and F-line light match each other. a first lens unit having a positive refractive power and being configured to move during focusing; a second lens unit having a negative refractive power and being configured to move towards the image side during zooming from a wide-angle end to a telephoto end; a third lens unit having a positive refractive power and being configured to move during zooming to compensate for the variation of an image plane caused by zooming; and a fourth lens unit having a positive refractive power and being configured not to move for zooming, the second and third lens units (U2, U3) constituting a system; characterized in that the second lens unit includes at least one positive lens and at least one negative lens, and that average values of Abbe number (ν) and relative partial dispersion (θ) of materials of the at least one negative lens (νna, θna) and average values of Abbe number (ν) and relative partial dispersion (θ) of materials of the at least one positive lens (νpa, θpa) and a focal length of the first lens unit and a focal length of the zoom lens at a telephoto end (ftele) satisfy the following conditions: - 10.0 × 10 - 3 < θpa - θna / νpa - νna < - 3.7 × 10 - 3 3.9 < ftele / f ⁢ 1 < 6.0; wherein the value of the secondary spectrum of axial chromatic aberration in the first lens unit can be effectively decreased by the value of the secondary spectrum in the second lens unit; and the zoom lens corrects chromatic aberration that may occur in each lens to the level as low as approximately 0, so that for each lens unit, the image forming positions of C-line and F-line light match each other.