AN AGENT FOR TREATING ULCER CONTAINING, AS AN EFFECTIVE COMPONENT, A PARTICULATE COMPOSITE HYDROTALCITE OBTAINED BY SOLIDLY DISSOLVING A SMALL AMOUNT OF ZINC IN THE PARTICULATE HYDROTALCITE, EXHIBITING EXCELLENT EFFECT FOR TREATING THE PEPTICULCER AND, FURTHER, WORKING AS A ZN-SUPPLYING AGENT. THE AGENT FOR TREATING THE ULCER IS REPRESENTED BY THE FOLLOWING FORMULA (1), (MgaZnb)1-xAlx(OH)2(An⁻)x/n.mH2O --- (1) WHEREIN An⁻ IS CO3² ⁻, SO4² ⁻, OR C1⁻, n IS 1 OR 2, AND x a, b AND m ARE VALUES THAT SATISFY THE FOLLOWING CONDITIONS, 0.18 ≦X ≦ 0.4, 0.1 ≦ a <; 1, 0 <; b ≦ 0.5, 0 ≦ M <; 1.