Nitrogen-containing 5-membered cyclic compounds represented by the general formula (I) and nontoxic salts thereof, wherein each symbol is as defined in the description. Because of having a cysteine protease inhibitory activity, the compounds of the general formula (I) are useful as preventives and/or remedies for inflammatory diseases, diseases caused by apoptosis, diseases caused by immune response error, autoimmune diseases, diseases caused by the degradation of biological constituent proteins, shock, circulatory error, blood coagulation error, malignant tumor, acquired immunological deficiency syndrome (AIDS), AIDS related complex (ARC), parasitogenic diseases, neurodegenerative diseases, lung disorder, bone resorption diseases, hyper endocrine diseases, etc.