To provide a medium telephoto lens which is compact and has proper performance levels.

This optical system is provided with a 1st lens group, having positive refractive power, a 2nd lens group having negative refractive power and a 3rd lens group having positive refractive power in this order starting from an object side, and the 3rd lens group is provided with a 3a-th lens group and a 3b-th lens group having positive refractive power, then the 3a-th lens group is constituted of one positive lens or one positive lens and one negative lens, and the 3b-th lens group is constituted of one or two positive lenses and one negative lens. Then, focusing is made, by moving the 2nd lens group on an optical axis and the blur of a photographic image is corrected, by making the 3b-th lens group displaced in an almost perpendicular direction to the optical axis. The optical system satisfies respective conditional expressions, if the focal distance of an i-th lens group is defined as (fi), and the focal distance of the entire system is defined as (f).