The present invention provides a substance which strongly inhibits human -amylase and is effective for the prevention and treatment of hyperglycemia, for example, diabetes and the diseases caused thereby, and an -amylase inhibitor and a prophylactic or therapeutic agent for hyperglycemia containing the said substance as active principle. Maltoligosaccharide derivatives represented by the following formula: ##STR1##

(wherein n is an integer of 0 to 2; m is an integer of 0 to 2; and X represents a hydrogen atom or a hydrophobic group) or their hydrates or physiologically acceptable salts are provided, and they are contained as active principle to prepare an -amylase inhibitor and a prophylactic or therapeutic agent for hyperglycemia such as diabetes.