PURPOSE: To obtain an α-glucosidase inhibitor containing a specific enopyranose derivative or its salt as an active ingredient, useful as a reagent for studying glucoprotein sugar chain processing and a medicine for treating viral diseases such as metastasis and AIDS, diabetes mellitus, etc.

CONSTITUTION: This α-glucosidase inhibitor contains an enopyranose derivative of formula I (R1 and R2 each is H or an alkyl; R3 is H or an acyl; R4 is OH, an acyloxy or R3 and R4 together form a single bond; X is H or an acyl) or its salt as an active ingredient. As the compound of formula I as the active ingredient, especially, an enopyranose derivative expressed by formula II or formula III or its salt is preferably used. In the compound of formula I, a combination in which R1 is H or an alkyl and R2 is an alkyl and X us H or an acyl is preferable. Further, the compound of formula I in which X is furan carbonyl or thiophene carbonyl is preferable. This α-glucosidase inhibitor is useful as a medicine for treating hyperlipemia, inflammatory diseases, autoimmune diseases, rejection in organ transplantation, allergic diseases, etc., especially as metastasis inhibitor.