An imaging optical system, which is designed in a small size with low manufacturing costs, aberrations such as chromatic aberration of the imaging optical system being appropriately corrected, is provided. The imaging optical system comprises a first lens having positive power, a second lens having positive power and a third lens having negative power with an aspherical surface in the image side, the aspherical surface being shaped in a concave shape facing toward an image side in a vicinity of the optical axis and shaped in a convex shape facing toward the image side in a surrounding of the optical axis. The imaging optical system is designed to satisfy a formula, ν3−ν2>10, where ν3 denotes an Abbe number of the third lens and ν2 denotes an Abbe number of the second lens.