An imaging optical system that includes, in order from an object side, a first lens having positive refracting power, a concave surface of which is directed toward the object side, a third lens having positive refracting power, a convex surface of which is directed toward an image side and a fourth lens having negative refracting power, wherein the second lens and the third lens are cemented together. Having such a configuration, an angle made by the incident light and the existing light to the cemented lens can be kept small, and accordingly, the generation of aberrations at the refracting surface and performance change of lenses when these are in a relative decentering position can be suppressed.