The present invention provides a postprandial GIP secretion inhibitor comprising potassium alginate as an active ingredient. The postprandial GIP secretion inhibitor of the present invention is useful as a medicine or a food product. The present invention also provides use of potassium alginate for the manufacture of a postprandial GIP secretion inhibitor. The present invention also provides a method for inhibiting postprandial GIP secretion, which comprises administering potassium alginate to a subject in need thereof or causing a subject in need thereof to consume potassium alginate.