An N-containing five-membered ring compound of formula (I) ##STR00001## wherein all symbols are the same as described in the specification, and a non-toxic salt thereof.The compound of formula (I) has an inhibitory activity against cysteine protease and therefore it is useful as an agent for the prophylaxis and/or treatment of inflammatory diseases, diseases induced by apoptosis, diseases induced by disorders of immune responses, autoimmune diseases, diseases induced by decomposition of proteins which compose organism, shock, circulatory system disorders, blood coagulation systems disorders, malignant tumors, acquired immune deficiency syndrome (AIDS) and AIDS-related complex (ARC), parasitic diseases, nerve degeneration diseases, pulmonary disorders, bone resorption diseases, endocrinesthenia, etc.