An optical system, in order from an object side to an image side, includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive or negative refractive power. The first lens unit is configured by a first partial lens unit and a second partial lens unit each having a positive refractive power. The first partial lens unit is configured by a positive single lens, and has at least one aspherical surface. The second partial lens unit is configured by a positive lens and a negative lens, and has a diffraction optical surface of a diffraction optical element rotationally symmetric with reference to an optical axis direction. The second lens unit is a focus lens unit that is movable in the optical axis direction. And a paraxial lateral magnification βasph meets a predetermined condition.