Arsenic (As) and cadmium (Cd) in rice grain pose threats to human health. Their concentrations in rice grain are sensitive to water managements such as flooding and drainage, and resultant changes in the soil redox potential. However, there has been a paucity of reports on the soil redox conditions under which uptake of As and Cd by rice and their concentrations in grain may be simultaneously suppressed. We carried out field experiments to find the optimal water management and soil conditions, in terms of dissolved ferrous iron (Fe(II)) concentration, for simultaneous reduction of As and Cd in the soil solution and rice grain. Dissolved As and Cd concentrations during pre-heading 3 weeks and post-heading 3 weeks, which exerted strong influence on the As and Cd concentrations in grain, showed distinct relationships with the dissolved Fe(II) concentration. In a given paddy field, the dissolved Fe(II) serves as an indicator of the soil redox condition, and the concentration within the range of 20–30 mg/L was found to be optimum for simultaneous reduction of dissolved As and Cd concentrations in the paddy field tested.