Even if the ratio between blue and green components of illumination light is changed, a plurality of types of blood vessels at different depths are reliably distinguished. A blue signal B, a green signal G, a red signal R is obtained by imaging the subject using a color CCD 44. A B/G image having a B/G ratio is generated. A superficial blood vessel extraction image is obtained by extracting a pixel, in which the B/G ratio is equal to or less than a boundary value Ls between the mucous membrane and the superficial blood vessel, from the B/G image. A medium-deep blood vessel extraction image is obtained by extracting a pixel, in which the B/G ratio is equal to or greater than a boundary value Ld between the mucous membrane and the medium-deep blood vessel. The boundary values Ls and Ld differ depending on the light amount ratio.
