The present invention provides an azulene derivative and a salt thereof, wherein an azulene ring is bonded to a benzene ring directly or via a lower alkylene which may be substituted with a halogen atom and the benzene ring is directly bonded to the glucose residue, and it is usable as a Na.sup.+-glucose cotransporter inhibitor, especially for a therapeutic and/or preventive agent for diabetes such as insulin-dependent diabetes (type 1 diabetes) and insulin-independent diabetes (type 2 diabetes), as well as diabetes-related diseases such as insulin-resistant diseases and obesity.