To cover sufficient optical performances in semi-wide range to medium telescopic range photographing, be suitable for a large-sized high definition image pickup device, and allow downsizing.SOLUTION: A large diameter imaging optical system comprises: a second lens group 102 composed of three lenses of two positive lenses L4, L5 and one negative lens L6 from an object OBJ side, and including on a concave surface a lens surface facing an aperture diaphragm STO; a third lens group 103 composed of five or less lenses including two negative lenses L7... and two positive lenses L8..., including on a concave surface a lens surface facing an aperture diaphragm STO, and having positive refractive power; and a first lens group 101 having positive refractive power, including at least one positive lens L2... from closest to the object OBJ side, in which a maximum lens quantity is set to be equal to or less than the total lens quantity of the second lens group 102 and the third lens group 103, and quantity difference between the positive lens L2... and the negative lens L1 is set to be equal to or less than one.SELECTED DRAWING: Figure 1