Recent studies have found that modern water supply systems played an important role in mitigating the mortality risks in major US cities in the early twentieth century. Modern water supply systems were installed also in Japanese cities during the interwar period. This study examines how the modern water supply system in Tokyo City reduced mortality risks in the interwar period. By employing a Bayesian disease mapping approach with a block-level lattice dataset of Tokyo for 1930, we found that wider access to purified water through water supply systems played an important role in mitigating mortality risks during the study period. Our estimation results show that clean water accounted for approximately 41.3 and 34.9 % of improvements in crude and child death rates, respectively, between 1921 and 1937 in Tokyo.