A histogram detecting unit detects a distribution of numbers of pixels by luminance level of a picked-up image signal. A maximum luminance value calculating unit calculates a maximum luminance value of the inputted image signal. A high luminance proportion calculating unit calculates a proportion of a high luminance signal out of all of the pixels composing one or a plurality of frames based on the detected distribution of numbers of pixels by luminance level. A knee point calculating unit calculates a knee point based on the calculated maximum luminance value and proportion of the high luminance signal. A knee processing unit generates a knee slope with a predetermined inclination starting at the calculated knee point. The knee processing unit does not change the value of the predetermined inclination, regardless of the value of the calculated knee point.