From an image signal made up of a series of image signal components representing a radiation image, an abnormal pattern appearing as an approximately circular pattern in the radiation image is detected. A first finding device finds a prospective abnormal pattern appearing in the radiation image by processing the image signal with a first filter capable of finding an approximately circular pattern. A second finding device finds a linear pattern appearing in the radiation image by processing the image signal with a second filter capable of finding a linear pattern or a linear pattern the width and the length of which are approximately equal to each other. A judgment device selects an enlarged prospective abnormal pattern region in an image area, in which both the prospective abnormal pattern and the linear pattern have been found, the enlarged prospective abnormal pattern region including the prospective abnormal pattern and extending to parts around the prospective abnormal pattern. The area and the maximum width of the linear pattern falling within the enlarged prospective abnormal pattern region are then calculated. From the area and the maximum width of the linear pattern, a judgment is made as to whether the prospective abnormal pattern is or is not a true abnormal pattern.