To clarify the cause of annual changes in methane (CH₄) emission from paddy fields incorporated with rice straw in a cold region of Japan, we measured methane flux from 1992 to 2013 (excluding 1997–2002). Annual methane emission was correlated with the cumulative average day temperature. Methane flux could not be clearly correlated with the soil redox potential (Eh) were not correlated when Eh was under −100 mV, which was the reducing condition. However, methane flux in June could be correlated with the average temperature in the same month. Methane flux decreased in June when Eh was over −100 mV, which was not the reducing condition. In contrast, methane flux in June correlated with methane flux in July, and methane flux in July decreased when Eh was over −100 mV in the same month. The results suggest that the duration of midseason drainage influences soil Eh in July and that methane emission may be reduced by prolonging the midseason drainage from paddy fields in cold regions of Japan.