PURPOSE: To provide the new stereoselective preparation by which each of the trisaccharide compounds can be prepared with lactose as a starting material and through a partially-alkylated glycal compound.

CONSTITUTION: This preparation of such a trisaccharide compound comprises: first, converting lactose into D-lactal; then, partially alkylating the D-lactal in the presence of dibutyltin oxide to form a lactal derivative represented by formula I (R is R'CH2 (R' is H, a 1-3C alkyl group, or the like) or CH2-CH=CH2; and R1 is H, R'CH2, or the like); converting the lactal derivative represented by formula I into a monoallyl compound in which each of R and R1 is completely protected with R'CH2; further, subjecting the monoallyl compound to azidonitration to stereoselectively convert it into an azide-lactose derivative represented by formula II (R is R'CH2); then, also stereoselectively converting the compound represented by formula II into a compound represented by formula III or IV (R1 is R'CH2; R is H or an allyl group; and R2 is a 1-4C alkyl or phenyl group; wherein the trichloroacetoimidate group has preferably an α-configuration); introducing a galactopyranosyl group into the compound represented by formula III or IV; replacing the resultant glycoside having this glucopyranosyl ring with a spacer to prepare the objective compound represented by formula V (wherein: S is a spacer; and X is N3 or NHCOCH3).