First and second white light is generated by excitations of phosphors with first and second laser beams having center wavelengths of 473 nm and 445 nm, respectively. The first and second white light is applied, in respective frames, sequentially to a region of interest in a subject. A color image sensor images the region of interest in the each frame. Based on a shift amount, calculated from green signals of first and second frames, between images, an image of a blue signal of the first frame is moved to be aligned with an image of a green signal and an image of a red signal of the second frame. After the alignment, an oxygen saturation image representing an oxygen saturation level of hemoglobin in blood is produced from the blue signal of the first frame and green and red signals of the second frame.
