Naphthalene derivatives represented by the formula (I): ##STR1## wherein the symbol ##STR2## --X-- represents --O-- or --S--, .dbd.Y-- represents .dbd.N-- or .dbd.CR.sup.5 --, R.sup.1, C.sup.2, R.sup.3, R.sup.4 and R.sup.5 represent hydrogen, halogen, alkyl and the like, R.sup.6 represents hydrogen, alkyl, aryl and the like, n represents an integer of 0 to 3, --- represents a single bond or a double bond, which are useful for reducing blood sugar and blood lipid levels are provided.