An imaging optical system includes, at the vicinity of the optical axis, a positive first lens element having an object-side convex surface, a negative second lens element having an object-side concave surface, a positive third lens element having an image-side convex surface, and a fourth lens element having an image-side concave surface. The fourth lens element is provided with aspherical surfaces on both sides such that a combined refractive power thereof has an increasingly weaker negative refractive power toward the outer periphery thereof. The following conditions (1) and (2) are satisfied: 38