An imaging optical system includes a positive first lens element having a convex surface on the object side, a negative second lens element having a concave surface on the image side, a third lens element, a positive fourth lens element, and a negative fifth lens element provided with at least one aspherical surface that has inflection points other than at an optical axis thereof. The following conditions (1) and (2) are satisfied: −0.80 60  (2), wherein r11 and r12 designate the radius of curvatures of surfaces on the object side and image side of the first lens element, respectively, and νd1 designates the Abbe number with respect to the d-line of the first lens element.