In an autofocusing zoom lens, a depth of focus ε is obtained based on an aperture value Fno of a variable diaphragm and a diameter of a permissible circle of confusion δ. A focal position variation amount Δsk for a predetermined positional displacement amount ΔFp0 of a focusing lens unit is obtained according to a position Fp of the focusing lens unit and a variable magnification state Zp. If a predetermined variation amount ΔFp of the focusing lens unit satisfies a first condition, the focusing lens unit is not driven. If the first condition is not satisfied and a second condition is satisfied, the focusing lens unit is driven based on a second autofocusing unit, and if the second condition is not satisfied, the focusing lens unit is driven based on a first autofocusing unit.