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, a fifth lens element, and a negative sixth 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:
0.86<f/f12<1.20 (1), and
−0.71<(r11−r12)/(r11+r12)<−0.20 (2),
wherein f designates the focal length of the imaging optical system, f12 designates the combined focal length of the first and second lens elements, and r11 and r12 designate the radius of curvatures of surfaces on the object side and image side of the first lens element, respectively.