To provide a variable power optical system and an optical instrument inexpensive but stabilized in the performance and capable of improving the productivity.

It has a 1st lens group having a negative refracting power and moving on the optical axis when changing its magnification, a 2nd lens group having a positive refracting power and moving on the optical axis when changing its magnification, and a 3rd lens group having positive refracting power but not moving when changing its magnification all arranged in this order from the object side. The above 1st lens group has glass lenses 101, 103 and a plastic lens 102 positioned adjacent to the above glass lens 101. The above 2nd lens group has a plurality of glass lenses 201, 203, 204 and a plastic lens 202 positioned adjacent to the above glass lens 201.