The invention provides a glucitol derivative having the function of reducing a blood sugar level and having preferable properties required of medicines, such as long-lasting drug activity; and a medicinal composition for use in the prevention or treatment of diseases attributable to hyperglycemia, such as diabetes, complications of diabetes, and obesity. The derivative is a compound represented by the formula (I): wherein m is an integer selected among 1-3; R1 to R4 each independently is optionally substituted alkyl, etc.; Ar1 is optionally substituted naphthyl; and A is optionally substituted heteroaryl, a prodrug of the compound, or a pharmaceutically acceptable salt of either. Also provided are a medicine, a medicinal composition, and the like each containing the compound.