An image pickup optical system according to the present invention includes: a first lens group having a positive power; a second lens group having a negative power; a third lens group having a positive power; and a fourth lens group having a positive power which consists of a front group with a negative power and a rear group with a positive power. The first lens group moves to the object side and a distance between each neighboring lens groups changes, for varying power of the image pickup optical system. One group of the front group and the rear group in the fourth lens group consists of one lens which moves substantially perpendicular to an optical axis for a shake compensation. The image pickup optical system fulfills the predetermined conditional formula according to a focal length of the first lens group.