In a special mode, a first frame image including a blue signal B1, a green signal G1, and a red signal R1 is captured in a first frame period. In a second frame period, a second frame image including a blue signal B2, a green signal G2, and a red signal R2 is captured. An oxygen saturation level calculator (86) calculates an oxygen saturation level of each pixel based on signal ratios B1/G2 and R2/G2, and sequentially outputs obtained special images on a monitor (14). Whenever the special image is produced, a signal ratio R2/R1 is calculated. When the signal ratio R2/R1 exceeds a threshold value, a warning sign "! !" (14a) is displayed. When the signal ratio R2/R1 is larger than a predetermined range S, a warning sign "HIGH StO2" (14b) is displayed. When the signal ratio R2/R1 is smaller than the range S, a warning sign "LOW StO2" (14c) is displayed.
