1. A method of detecting suspected anomalous shadows, comprising the steps of obtaining radiation-image data representing a radiation image obtained of a target subject by an image obtaining means, detecting, based on said obtained radiation image data, the suspected anomalous shadows contained within said radiation image by performing an anomalous shadow detection process utilizing a predetermined detection parameter, further comprising the steps of obtaining phantom-image data representing a radiation image obtained by said radiation image obtaining means of a standard phantom having a shadow pattern formed of a plurality of evaluative models each of which corresponds to a different detection level, outputting said obtained phantom image data by use of an output means, and setting as the value of the detection parameter a threshold value obtained by performing an image quality evaluation based on the phantom-image data outputted by the output means.