PURPOSE: To inexpensively provide a compact zoom optical system which is constituted so that respective aberration is excellently corrected and whose constitution is simple.

CONSTITUTION: The zoom optical system constituted of three lens groups I-II moved at a zooming time is constituted of two parts of a common lens group part consisting of the intermediate lens group II and the rear lens group III and a dedicated lens group part consisting of the front lens group (one of I-I") designed corresponding to the common lens group part and coupled with the common lens group. By selecting the front lens group (one of I-I") according to a desired variable power area, the zoom optical system provided with the desired variable power area out of the plural variable power areas whose zoom ratio are different is selectively realized. In such a case, the number of lens groups included in the dedicated lens group part whose variable power area should be selected is set to the minimum. That means, the number of lens groups included in the common lens group part is set as large as possible. As the result, a developing term is shortened and a cost is reduced.