An inter-frame difference generation circuit generates, in every pixel unit, a difference level of an image between a present frame and a frame before one frame period. A histogram generation circuit generates a difference histogram indicating a frequency distribution of difference levels between the frames in a state in which a difference corresponding to a pixel including predetermined or more image changes is removed from generated difference levels. An erroneously detected histogram exclusion judging section excludes a histogram which is inappropriate as a histogram for use in deciding a noise amount among histograms generated by the histogram generation circuit from the histograms generated by the histogram generation circuit. A noise amount judging section decides the noise amount based on the histograms remaining without being excluded by the erroneously detected histogram exclusion judging section. Therefore, noises included in a dynamic image are detected with a good precision.