The present invention discloses an image pickup optical system which has a first lens unit with positive optical power, a second lens unit with negative optical power, a third lens unit with positive optical power, and a fourth lens unit in order from an object side to an image side, where the second lens unit moves toward an image side and the third lens unit moves toward an object side respectively during focusing from an infinite object to a short distance object. The first lens unit has a first lens sub-unit, and a second lens sub-unit in order from an object side to an image side, and corrects a blur of a taken image by moving the second lens sub-unit so as to have a component in a direction orthogonal to an optical axis. Thereby, the present invention realizes the image pickup optical system which can always obtain a favorable image even if a shake of a camera or the like arises.