It is preferable that for an image forming optical system a basic structure which includes a first lens group having a negative refracting power, which is made of one cemented lens component, a second lens group having a positive refracting power, a third lens group having a positive refracting power, which is made of one lens component, and is movable for focusing, and a fourth lens group which is made of one lens component having an aspheric surface, and at the time of zooming from a wide angle end to a telephoto end, the second lens group moves only toward the object side, and the first lens group and the third lens group move while following a trajectory of movement different from a trajectory of movement of the second lens group is adopted, and that the image forming optical system satisfies predetermined conditional expressions.