A variable power optical system composed of a first optical part (G1) for refraction, a second optical part (G2) for reflection or transmission, and a third optical part (G3) for refraction, with the first optical part being movable with respect to the second and third optical parts to change the focal length of the entire system, wherein the second optical part or the third optical part has a guide 4 which is in contact partly with the first optical part in such a way as to restrict its moving direction and the first optical part is movably held between the second optical part and the third optical part. This structure eliminates the necessity of separately forming the guide to move G1. The guide 4 is formed from a synthetic resin integrally with the stationary optical part (G3).