To provide a diffractive optical element which has almost constant diffraction efficiency for a wide wavelength range and which can be easily manufactured.

A first region and a second region with uniform distribution are settled in a diffraction grating having a step-like cross-sectional form in a single period. The level difference in one period of the second region is twice as the level difference in one period in the first region and the designed wavelength in the second region is twice as the designed wavelength in the first region. The number of steps in one period of the second region is same as the number of steps in one period of the first region and the horizontal width in the step-like part is made identical for all steps so that the diffraction efficiency for the designed wavelengths in the first and second regions is optimized. By controlling the ratio of the total area of the first region to the total area of the second region, the diffraction efficiency in the whole wavelength range from the designed wavelength in the first region to the designed wavelength in the second region is controlled to almost constant.