The image forming optical system of the present invention comprises, in order from an object side, a first lens which is meniscus lens having positive refracting power and a concave surface directed toward an object side, an aperture stop, a second lens having positive refracting power and a convex surface directed toward the object side, a third lens which is meniscus lens having negative refracting power and a convex surface directed toward an image side, and a fourth lens having positive refracting power.