To provide a focus adjustment system using a diffraction optical element by deciding diffraction phase profiles of a first diffraction optical element and a second diffraction optical element with a specified equation related to a position on a surface.

When an f value is 1, and a whole object/image view is either one of 0.5 or 1 μm, an optical system is provided with a spot size of 1 μm or below and Strehl ratio exceeding 0.9. A divergent light beam 14 is diffracted by the first diffraction optical element 16, and diffracted light 18 is faced to the second diffraction optical element 20. The second diffraction optical element 20 diffracts the diffracted light 18 from the first diffraction optical element 16, and faces a diffracted beam 22 obtained as the result to a spot 24 on an image surface 26. Since the first, second diffraction optical elements 16, 20 are circular symmetry, the diffraction phase profile Φ is shown by the equation: Φ(ρ)=c1ρ2+c2ρ4+c3ρ6+c4ρ8+c5ρ10 on the position ρ in the radial direction. Where, cx is a polynominal coefficient.