An image forming optical system of the present invention is characterized by comprising three groups having, in order from an object side to an image side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power, or four groups with one more lens group added to the image side, the first lens group comprises one lens component facing its concave surface toward the object side, the second lens group comprises a single lens having a positive refractive power and a cemented lens having a negative refractive power as a whole, and the third lens group comprises a lens component having a positive refractive power, wherein upon zooming from the wide-angle end toward the telephoto side, the amount of movement of the first lens group in the optical axis direction in an area, where the focal length of the entire image pickup optical system is 3.4 times or more with respect to at the wide-angle end, satisfies the following conditional expression (1): −1≦(G1(W)−G1(H))/fw≦0.5  (1).