The impact of alcohol on the risk of stomach cancer is controversial. Although aldehyde dehydrogenase 2 ( ALDH2 ) Glu504Lys (rs671) polymorphism has a strong effect on acetaldehyde metabolism, little is known about its impact on stomach cancer risk when combined with alcohol drinking. This case-control study included a total of 697 incident stomach cancer case subjects and 1372 non-cancer control subjects who visited Aichi Cancer Center between 2001 and 2005. We estimated odds ratios (OR) and 95% confidence intervals (CI) for ALDH2 genotypes and alcohol consumption using logistic regression models after adjustment for potential confounders, including Helicobacter pylori infection. The ALDH2 504Lys allele was associated with the risk of stomach cancer, with adjusted ORs of 1.40 (95% CI, 1.11–1.76) for Glu/Lys and 1.73 (1.12–2.68) for Lys/Lys compared with Glu/Glu. Heavy drinking was associated with risk (OR 1.72, 1.17–2.52) after adjustment for ALDH2 genotype and other confounders. Moreover, ORs for heavy drinking were 1.28 (0.77–2.12) for those with ALDH2 Glu/Glu and 3.93 (1.99–5.79) for those with the ALDH2 Lys allele relative to non-drinkers with the Glu/Glu genotype ( P for interaction = 0.0054). In conclusion, ALDH2 and alcohol drinking showed interaction for risk factors of stomach cancer, indicating that acetaldehyde plays a role in stomach carcinogenesis.