A benzene-fused heteroring derivative of formula (I) 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 immune diseases (autoimmune diseases, infectious diseases, etc.), inflammatory diseases (inflammatory bowel diseases, multiple cerebrosclerosis, arthritis, etc.), nerve degeneration diseases (Alzheimer's disease, muscular dystrophy, etc.), bone resorption diseases (osteoporosis, etc.), respiratory system diseases, diabetes, shock, etc.