To enhance the degree of freedom of diffracting means design by giving a specific sag quantity to the surface where a diffracting means is formed.

A virtual high-refractive-index lens which touches the surface of an existing lens is assumed and a virtual cemented lens is constituted. For design data, the angle θ (h) of a light beam in the existing lens to the optical axis which corresponds to the height (h) from the optical axis and the length L(h) in the virtual lens along its optical path are found. Then the sag quantity S(h) given by an equation (distance between the contact plane of the lens at the intersection of the lens and optical axis and the lens surface) is calculated. In the equation, nb is a virtual high refractive index and Q(h) is the phase delay quantity of a wave front when the wave is transmitted through the virtual lens. Then (n) is the refractive index of the existing lens material and M(h) is a correction term. Further, F(h) represents the swell shape of the diffracting means corresponding to a phase grating. A period function P(x) represents the phase delay quantity by the phase grating and D(h) shows the shape of the virtual cemented surface of the virtual cemented lens.