A plurality of image signals are obtained, which represent a radiation image of a single object or radiation images of the single object, and which have different frequency characteristics, each of the image signals being made up of a series of image signal components. The image signal components of the plurality of the image signals, which image signal components represent corresponding picture elements, are weighted with predetermined weight factors. The weighted image signal components of the plurality of the image signals, which image signal components represent corresponding picture elements, are added to one another, and an addition signal is thereby obtained. The value of the weight factor with respect to the frequency components, which have a low signal-to-noise ratio, is rendered smaller than the value of the weight factor with respect to the frequency components, which have a high signal-to-noise ratio, in accordance with the frequency characteristics of each of the image signals.