This article studies the interrelation between spot and futures prices in the two major rice markets in prewar Japan from the perspective of market efficiency. Applying a non-Bayesian time-varying model approach to the fundamental equation for spot returns and the futures premium, we detect when efficiency reductions in the two major rice markets occurred. We also examine how government interventions affected the rice markets in Japan, which colonized Taiwan and Korea before the Second World War, and argue that the function of rice futures markets crucially depended on the differences in the structure of rice spot markets. Initially the increased volume of imported rice of a different variety from domestic rice disrupted the rice futures markets. Then, government intervention in the rice futures markets failed to improve the disruption. Changes in colonial rice cropping successfully mitigated the disruption, and colonial rice was promoted in order to unify the different varieties of inland and colonial rice.