Comprising a positive first lens group G1 disposed at a most object side, a negative intermediate group G2 disposed at an image side of the first lens group G1, a positive focusing group G3 disposed at an image side of the intermediate group G2, the focusing group G3 being moved upon focusing, and a positive image side group G4 disposed at an image side of the focusing group G3; upon varying magnification, a distance between the first lens group G1 and the intermediate group G2, a distance between the intermediate group G2 and the focusing group G3 and a distance between the focusing group G3 and the image side group G4 being varied; and a predetermined conditional expression being satisfied. Whereby a variable magnification optical system having excellent optical performance and a focusing lens group reduced in weight in order to attain high speed focusing operation, and others are provided.