a stepped structure having a concentric pattern on at least one side, wherein the stepped structure further comprises a step depth d, wherein d is defined by the formulas:

d=m_o×λ_o/(n_o−1); d(n₁−1)/λ₁=m₁+α; and d(n₂−1)/λ₂=m₂+β

where m_o is an integer, λ_o is a wavelength for a high-density recording medium, and n_o is a refractive index of a diffractive optical element at the wavelength λ_o; m₁ is an integer, λ₁ is a wavelength for a first low-density recording medium, n₁ is a refractive index for a diffractive optical element, and −0.1<α<0.1; m₂ is an integer, λ₂ is a wavelength for a second low-density recording medium, and −0.1<β<0.1, and the first low-density recording medium has a higher recording density than the second low density recording medium,

wherein a phase difference is generated between two adjacent steps of the stepped structure represented by d(n₀−1)/(λ_o+Δλ), when a light beam with a wavelength of (λ_o+Δλ) is incident on the chromatic aberration-correcting element,

wherein

d(n₀−1)/(λ_o+Δλ)≈{d(n_o−1)/λ_o}(1−Δλ/λ_o)=m_o(1−Δλ/λ_o) (λ_o>>Δλ), and

wherein steps of the concentric pattern increases by one order with regard to a radius r of the chromatic aberration-correcting element when a waterfront aberration increases by −m_o(1−Δλ/λ_o).