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Methane formation in flooded soil and emission from rice plants were examined by pot culture experiments. Rice straw treatment resulted in a two-fold increase in methane formation in flooded soil and a six-fold increase in methane emission rate from rice plants compared to that obtained with a straw compost treatment. Methane concentration in the surface water above the soil was extremely low as compared to that in the soil water. Temperature affected methane formation in flooded soil. Large amounts of methane formed in flooded soil when the redox potential was below −200 mV. In the high temperature (31°C) treatment, methane concentration in vegetated soil was about one-third of that in unvegetated pots; methane emission rate was 20 times higher in vegetated than in unvegetated pots. These results indicate that rice plants have a large capacity to transport methane from the rhizosphere to the atmosphere. Methane transport capacity of rice plants depends mainly on plant size. The present results indicate that the seasonally appearing peak of methane flux from paddy fields results from a combination of flooded soil and rice plant growth.