The image forming optical system comprises, in order from an object side, a first lens which is positive meniscus lens having a convex surface directed toward an object side, an aperture stop, a second lens which is positive meniscus lens having a convex surface directed toward an image side, a third lens which is positive meniscus lens having a convex surface directed toward an image side, and a fourth lens which is negative lens, wherein at least one of surfaces of the fourth lens is aspherical and the following condition is satisfied:

where m represents the power of the fourth lens at the position of the maximum light height and p represents the power of the fourth lens at the position of the praxis. The third lens and the fourth lens are made of plastic material and the following condition is satisfied:

where 3 represents Abbe's number of the third lens and 4 represents Abbe's number of the fourth lens.