A process for producing an azulene derivative useful as a Na.sup.+-glucose cotransporter inhibitor, which is high in yield, is simple in operation, is low in cost, is suited for environmental protection, and is advantageous industrially, the process being characterized by reducing and deprotecting at least one compound selected from penta-acyl compounds and tetra-acyl compounds or salts thereof to obtain a C-glycoside compound; and a useful intermediate for synthesis of such an azulene derivative, obtained in the course of the above process.