To provide an optical system for not only improving diffraction efficiency in a wide wavelength band but also improving the diffraction efficiency from on-axis light to off-axis light.

In this optical system provided with a diffraction optical element composed of laminated two optical materials N2 and N3 for which the relief pattern of a diffraction grating is formed on the boundary surface r4*#, the diffraction optical element is turned to a lens provided with a convergence or divergence action. Then, the boundary surface r4*# is positioned relatively separately from the pupil or diaphragm of the optical system and the diffraction grating cross sectional shape of the relief pattern is turned to a blaze shape in a center area centering a lens optical axis AX and turned to a triangular shape in a peripheral area centering the lens optical axis AX.