Broadband light BB and narrowband light NB are simultaneously irradiated to a subject. A blue signal B, a green signal G, and a red signal R are obtained by imaging the subject using a color CCD 33. A base image is generated from the signals B, G, and R of three colors. 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 each observation mode.
