Essential oil from the leaves of Tateyamasugi (Cryptomeria japonica) exhibited strong inhibitory activity on ulceration induced by HCl/ethanol, HCl/aspirin, water-immersion stress and pylorus-ligation. We separated the antiulcer compounds from cedar essential oil by use of distillation and chromatography. As a result, terpinen-4-ol, a monoterpene, and elemol, a sesquiterpene, were isolated as active compounds. The antiulcer activity of the former was more potent than that of the latter. Terpinen-4-ol was a mixture of optical isomers and each possessed potent antiulcer activity. Secretion of gastric juice and output of acid and pepsin activity were lowered by terpinen-4-ol. These results suggest that terpinen-4-ol isolated from cedar essential oil could be a valuable antiulcer agent.