In an image forming optical system which includes in order from an object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3 having a positive refractive power, and a fourth lens group G4, and which includes maximum of five lens groups, and at the time of zooming from a wide angle end to a telephoto end, when a focal length of the overall image forming optical system is in a range of 1.2 fw to 1.8 fw, the first lens group G1 moves to be positioned more toward the object side than a position at the wide angle end, in a rectangular coordinate system in which, a horizontal axis is let to be νd(LA) and a vertical axis is let to be nd(LA), when a straight line expressed by nd(LA)=a×νd(LA)+b(LA)(provided that a=−0.0267) is set, the image forming optical system satisfies a predetermined conditional expression.