1. A computer aided diagnostic system, comprising: a processor configured to detect a cancer candidate by automatically extracting a field of interest based upon a simple X-ray image acquired by a simple X-ray device, and extracting the cancer candidate in the field of interest, the simple X-ray image being defined over a first direction and a second direction perpendicular to the first direction; automatically determine whether a sick portion candidate corresponding to the cancer candidate exists in any of a plurality of X-ray CT images acquired by an X-ray CT device and corresponds to a position of the detected cancer candidate; and cause a marker extending in a third direction intersecting with the first and second directions to be displayed on a displayed X-ray CT image when it is determined that the sick portion candidate corresponding to the cancer candidate does not exist in any of the X-ray CT images, the marker indicating a region in which a sick portion candidate might exist.