In an optical system (ZL) capable of imaging in a plurality of object side media (for instance, air and water) whose refractive indexes are different from each other, the optical system is composed of, in order from an object, a front lens group (G1) whose position in an optical axis fixed, and a rear lens group (for instance, G2 to G4), and the rear lens group comprises at least two moving lens groups (for instance, G2 and G3), and at least two moving lens groups in the rear lens group are moved along the optical axis upon changing between object side media.