A maltooligosaccharide derivative very useful, for example, as a precursor of a modified oligosaccharide derivative having one or more modifying groups at the 6-position or the 4- and 6-positions of the non-reducing end glucose residue which can be effectively used as a substrate for determining .alpha.-amylase activity or an intermediate thereof, can be produced by reacting a compound of the formula: ##STR1## with a compound of the formula: ##STR2## to convert at least the hydroxyl groups at the 4- and 6-positions of non-reducing end glucose residue of the compound of the formula (1) into a cyclic boric acid ester, acylating the reaction product, and then treating the acylated product.