[en] In this paper, we propose a minimum directional difference filter (Min-DD filter) for enhancing abnormal shadows in chest X-ray images. This filter is constructed by a set of second order difference filters and its output is the minimum value of all the 2nd order difference filters used here. It can enhance only round shadows, while suppressing output for linear shadows. We evaluate the performance of this filter by the experiment with artificial images. Then we improve this filter to extract abnormal shadows in the peripheral region of lung more effectively, and discuss the ability of this improved filter. (author)