In Hokkaido, Japan, in order to investigate the etiological relation between gastric cancer and lifestyle, a case-control study was conducted, employing 242 cases with this cancer and 484 controls matched for sex, age, and place of residence. Eating meals within a short time, eating until full, irregularity in the time for supper, having dentures, and lack of teeth showed high odds ratios. Putting an excess digestive burden on the stomach by insufficient chewing promotes the development of this cancer. Stress, long working time, irregularity in sleeping time, and gray hair in males showed high odds ratios. A stressful life may be promotive for this cancer through depressed immunoresistance. Logistic regression analysis showed that those who ate meals rapidly, had strong stress and had lost many teeth were at high risk. The linear trend of the dose-response relationship between the number of teeth lost and the odds ratio was highly significant. To avoid stress, to chew foods sufficiently, and to maintain the health of the oral cavity can help to prevent gastric cancer.