An imaging optical system smaller than conventional systems yet with well-corrected aberrations, comprises five lenses L1-L5 having refractive powers, in order from the object side: positive, negative, positive, positive, and negative, with an aperture at an endmost object-side position. In one embodiment, L1 is biconvex, L2's object-side surface is convex, L5's object-side surface is convex, and 1.0≦f3/f1≦4.9, f1 and f3 being the focal lengths of L1 and L3, respectively. In a variant thereof, L2 is a meniscus lens and L5's object-side surface is concave. In another embodiment, L1 is glass, and −4.0