The image forming optical system includes, in order from the object side, a meniscus, a first lens having positive refracting power with a convex surface directed toward the object side, an aperture stop, a meniscus, a second lens having negative refracting power with a concave surface directed toward the object side, a third lens having positive refracting power with a convex surface directed toward the image side, and a biconvex, fourth lens having positive refracting power. The fourth lens has at least one aspherical surface and satisfies the condition: 1.0<.phi.m/.phi.p<50.0 where .phi.m and .phi.p represent power of the fourth lens at a position of the maximum light height and at the paraxial position, respectively. Also, the following condition is satisfied: 0.3