PROBLEM TO BE SOLVED: To provide an imaging optical system in which entire length remains stationary when focusing with a wide angle of view while securing size reduction and an amount of peripheral light.SOLUTION: An imaging optical system comprises: a front side lens group LF comprising a negative first sub lens group L1 and a positive second lens group L2; an aperture diaphragm; and a positive rear side lens group LR comprising a third sub lens group L3 and a fourth sub lens group L4 having positive refractive power, and satisfies following conditional expressions (1), (2) and (3); -140%≤DT≤-7% (1), -2.2≤R_L3f/f≤-0.25 (2) and 0.8≤R_L1f/f≤5.5 (3), where DT=(IHω-f×tanω)/(f×tanω)×100%, ω denotes a maximum half angle of view of the imaging optical system, IHω denotes distance from an optical axis of a point where a principal light beam at a maximum half angle of view ω intersects an imaging plane, f denotes focal length of an entire system, R_L3f denotes
