A generation method of a digital hologram includes steps of emitting coherent light from a coherent light source, imaging a hologram that is an interference pattern of an object beam and a reference beam due to the emission light from the light source, and setting a plurality of wavelengths of the illumination light that generates the hologram detected by the detector, and wherein the plurality of wavelength are specified by the wavelength setting step based on a magnification percentage X of a conjugate image set up by a user not to disturb visibility of an image when a real image and the conjugate image reconstructed by a predetermined calculation means relative to structures of observation targets are superimposed to a corresponding real image so that a shortest wavelength λmin and a longest wavelength λmax satisfy the expression λmax/λmin≥(1/X+1).