Cultivation of forage rice using treated wastewater for irrigation can contribute to food security in Japan which imports most of fertilizer and livestock feed. In this study, we investigated the greenhouse gases (GHG) emitted from forage rice cultivation in a real paddy field irrigated with treated municipal wastewater (test field) and compared them with those in the conventional rice paddies cultivated with chemical fertilizers (control field). The total emissions of GHG, integrating methane and nitrous oxides, were 7506 ± 3588 kgCO₂-eq/ha in the test field, which were not significantly different from that (7035 ± 3207 kgCO₂-eq/ha) in the control field. However, in the more comprehensive approach, the GHG emissions could be reduced by 819 kgCO₂-eq/ha in the test field, where treated wastewater is reused instead of fertilizer application, mainly due to the reduction of CO₂ emission during the production and transportation of the chemical fertilizers used in the control field.