At a first time point, a light amount adjusting unit stores the brightness of illumination light as a first brightness, and stores the light intensity of each color component of the illumination light as a first light intensity. A gain adjusting unit sets a gain value of at least one of the individual color components to be a maximum value, and stores the gain value as a first gain value. At a second time point after the first time point, the light amount adjusting unit calculates a ratio between a second light intensity and the first light intensity. The gain adjusting unit corrects the gain values based on the ratio between the second light intensity and the first light intensity.