Tissue information of a desired deep portion of a biological tissue based on a spectral image obtained from signal processing is adjusted to image information in a color tone suitable for observation. Outputs of a matrix computing section 436 are respectively connected to integrating sections 438a to 438c, and after integrating computation is performed for them, color conversion computation is performed for respective spectral image signals ΣF1 to ΣF3 in a color adjusting section 440, spectral color channel image signals Rch, Gch and Bch are created from the spectral image signals ΣF1 to ΣF3, and images of the spectral color channel images Rch, Gch and Bch are sent to a display monitor 106 via a switching section 439.