A white light from a lamp 11 passes through R, G, and B filters of a rotating filter 14 and turns into broadband R, G, and B frame sequential illumination lights. The lights are applied to a living mucous membrane in a body cavity through an electronic endoscope 2, and sequential image pickup is performed by a CCD 25. Output signals from the CCD 25 are sequentially switched by a selector in a filter circuit 36. An R signal, a G signal, and a B signal pass through the filter circuit 36 through no medium, a BPF, and an HPF, respectively, and are separated into signal components corresponding to spatial frequency components of living mucous membrane structures. The separation leads to generation of image signals of an image which allows easy identification of the living mucous membrane structures.
