There is provided an imaging optical system including: a first lens element having a convex object-side surface; a second lens element having an object-side planar surface in contact with an image-side surface of the first lens element and an image-side planar surface; and a third lens element having an object-side surface in contact with an image-side surface of the second lens element, wherein the object-side surface of the first lens element and an image-side surface of the third lens element are aspherical, and the third lens element has a point of inflection formed within an effective aperture thereof such that the image-side surface is convexed toward an image plane at a central portion to have positive refractive power and concaved toward the image plane at a peripheral portion to have negative refractive power.