A method for preventing or treating gastritis or peptic ulcer, the method comprising administering to a mammal an ionically bonded anthocyanin-alginic acid complex in a therapeutically effective amount for preventing or treating gastritis or peptic ulcer, wherein the anthocyanin is obtained from a fruit of aronia and is a mixture comprising cyanidin-3-galactoside, cyanidin-3-glucoside, cyanidin-3-arabinoside and delphinidin-3-glucoside, and wherein the weight ratio of anthocyanin and alginic acid in the complex is 1:10 to 50.