Providing a compact rear-focus optical system having excellent optical performance, an imaging apparatus, and a method for focusing the rear-focus optical system. The system includes, in order from an object, a first lens group G1 having positive refractive power, an aperture stop S, and a second lens group G2. At least a portion of the second lens group G2 is movable along an optical axis of the rear-focus optical system upon focusing. Given conditional expressions are satisfied.