A diagnostic system comprises a spectral image pickup means that picks up a spectral image in a predetermined wavelength region in a body cavity and obtains spectral image data, an image processing means that obtains, from the spectral image data, an index-value for discriminating between a diseased portion and a healthy portion, and generates and outputs an indicator image based on the index-value, and a monitor on which the indicator image is displayed, wherein, for each pixel of the spectral image, the image processing means defines β obtained by a predetermined expression as the index-value, while using the spectral image data P1 at a first wavelength which is around a wavelength of 542 nm, the spectral image data P2 at a second wavelength which is around a wavelength of 558 nm and the spectral image data P3 at a third wavelength which is around a wavelength of 578 nm.