An imaging optical system consisting of a positive or negative first lens group (G1), an aperture diaphragm (S), and a positive second lens group (G2), in that order from the object side, wherein said first lens group (G1) includes a negative lens element provided closest to the object side within said first lens group (G1), wherein the negative lens element that is closest to the object side includes an aspherical surface on the object side thereof, said aspherical surface including a paraxial convex surface convexing toward the object side, a paraxial curvature that is the greatest within the effective aperture, and a portion within the effective aperture that has a curvature that is less than 1/2 of the paraxial curvature, and characterized in that the following conditions , , and are satisfied: R 1 / f < 1.35 D 1 / f > 0.4 − 2.5 < f 1 / f < − 1.3 and V > 56 wherein f designates the focal length of said imaging optical system, f1 designates the focal length of the negative lens element provided closest to the object side within said first lens group (G1), R1 designates a paraxial radius of curvature of a surface on the object side of said negative lens element provided closest to the object side within said first lens group (G1), D1 designates a thickness, along the optical axis, of said negative lens element provided closest to the object side within said first lens group (G1), and V designates the Abbe number with respect to the d-line of a lens element (21, 21', 21", 21A, 21B, 21C, 21D), within said second lens group (G2), that is provided closest to the aperture diaphragm (S), in that the following condition is satisfied: − 0.45 < f 1 / fg 1 < 1 wherein f1 designates the focal length of said negative lens element provided closest to the object side within said first lens group (G1), and fg1 designates the focal length of said first lens group (G1), and in that said first lens group (G1) includes a positive lens element behind said negative lens element provided closest to the object side within said first lens group (G1). f designates the focal length of said imaging optical system, f1 designates the focal length of the negative lens element provided closest to the object side within said first lens group (G1), R1 designates a paraxial radius of curvature of a surface on the object side of said negative lens element provided closest to the object side within said first lens group (G1), D1 designates a thickness, along the optical axis, of said negative lens element provided closest to the object side within said first lens group (G1), and V designates the Abbe number with respect to the d-line of a lens element (21, 21', 21", 21A, 21B, 21C, 21D), within said second lens group (G2), that is provided closest to the aperture diaphragm (S), in that the following condition is satisfied: f1 designates the focal length of said negative lens element provided closest to the object side within said first lens group (G1), and fg1 designates the focal length of said first lens group (G1), and in that said first lens group (G1) includes a positive lens element behind said negative lens element provided closest to the object side within said first lens group (G1). <EOS>