In an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the negative lens group is disposed at an image side of the of the aperture stop, the negative lens group has a cemented lens which is formed by cementing a plurality of lenses, and in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be .nu.d, when a straight line indicated by Nd=.alpha..times..nu.d+.beta. (where, .alpha.=-0.017) is set, Nd and .nu.d of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1), and a line when an upper limit value is in a range of the following conditional expression (1), and an area determined by following conditional expressions (2) and (3). 1.45<.beta.<2.15 (1) 1.30