A zoom optical system suited for use in an electronic image pickup apparatus comprising a first negative lens unit, a second positive lens unit and a third positive lens unit, in which a magnification is changed from a wide position to a tele position by moving at least the first lens unit and the second lens unit so as to change an airspace between the first lens unit and the second lens unit and widen an airspace between the second lens unit and the third lens unit, the second lens unit comprises a biconvex lens element and a cemented meniscus lens component having a convex surface on the object side, and each lens unit has a small thickness.