A zoom optical system includes a front-side lens unit, an intermediate lens unit, and a rear-side lens unit. The front-side lens unit includes a first front unit and a second front unit, and a distance between the first front unit and the second front unit is wider at a telephoto end than at a wide angle end. The intermediate lens unit includes a first intermediate unit and a second intermediate unit, and a distance between the first intermediate unit and the second intermediate unit is narrower at the telephoto end than at the wide angle end. A distance between the second intermediate unit and a lens unit adjacent to the second intermediate unit varies. The second intermediate unit moves at the time of focusing, and the following conditional expressions (1) and (2) are satisfied: 0.9≤LTLT/LTLW≤1.17  (1) 4.2≤KMBT≤20.0  (2).