PURPOSE: To obtain a new compound useful as an intermediate capable of simply synthesizing a new maltose derivative which is readily subjected to introduction of amino group to the 2'-position, sulfation at the 6'-position and glycosylation at the 4'-position and suitable for synthesizing a repeated disaccharide constituting heparin.

CONSTITUTION: A compound of formula I (R1 is protecting group; R2 to R4 are H, acyl, etc.; R5 to R8 are H, alkyl, etc.) such as phenyl-2',3:4',6'-di-0- benzylidene-a-maltoside. The compound is obtained by reacting a maltose derivative of formula II (R9 is protecting group) with an acetal compound or a ketal compound of formula III (R10 and R11 are H, phenyl, etc.; R12 and R13 are H or cycloalkyl) (e.g. benzaldehyde dimethyl acetal or 2,2-dimethoxypropane) preferably in the presence of an acid catalyst such as p-toluenesulfonic acid. The reaction is preferably carried out in an aprotic organic solvent such as OMF at pH≤4 at 0-100°C for 1-10 hours.