An image pickup optical system includes a first-lens unit having negative refractive power; a second-lens unit having positive refractive power and being disposed closer to an image side than the first-lens unit; an aperture; and a diffraction optical part provided on the joint surface of a first-cemented lens closer to the image side than the aperture. The focal length of the whole system, the Abbe number of the negative lens of a second-cemented lens closer to the image side than the aperture, the focal length within the air, the Abbe number of a negative lens closer to an object side than the aperture, the average value between the curvature radius closest to the object side and the curvature radius closest to the image side of the first-cemented lens including the diffraction optical part, the curvature radius of the diffraction optical part, and so forth, are set appropriately.