Abstract: Galactosyl maltooligosaccharide derivatives represented by formula (1): ##STR1## wherein at least one of X.sub.1 and X.sub.2 represents a galactosyl group and the other represents a hydrogen atom, R represents a hydrogen atom or a substituted or unsubstituted phenyl group, and n is an integer of from 2 to 5. The derivatives are prepared by reacting a sugar with a galactosyl group with a maltooligosaccharide represented by formula (2): ##STR2## R represents a hydrogen atom or a substituted or unsubstituted phenyl group, and n is an integer of from 2 to 5; in the presence of .alpha.-galactosidase or .beta.-galactosidase. The derivatives are used for measurement of .alpha.-amylase activity.