Digital image signals are detected from radiation images of an object constituted of bones and soft tissues. A subtraction process is carried out with a predetermined parameter on the digital image signals, and a difference signal representing an image of part of the object is obtained. Each of images of a phantom, which has part exhibiting an equivalent radiation absorption coefficient to bones and/or part exhibiting an equivalent radiation absorption coefficient to soft tissues, is read out together with each object image. A value of the parameter is found, with which the image of either part of the phantom becomes erased in a subtraction image of the phantom obtained from subtraction processing. The value of the parameter, which has thus been found, is determined as an optimum value of the parameter. Subtraction processing is then carried out by using the optimum value of the parameter.