Dynamics of dissolved methane concentration in rivers are explained by a decrease due to methane emission to the atmosphere and methane oxidation and an increase due to methane loading from the inside (mainly riverbed) and outside the river system. To reduce methane emissions from rivers, it is necessary to reduce both loadings, but for practical solutions, it is necessary to quantify the internal and external loadings, respectively. However, it is difficult to quantify both loadings, and there are limited studies of direct loading estimation. Therefore, we attempted to estimate the methane loading in a river by mass balance. In this study, we estimated the internal loading of methane in a river by using a stream reach with no tributary inflow and almost no flow change. The sampling was conducted in 2022 on the Kokai River, a tributary of the Tone River, Japan. We measured dissolved methane concentration and oxidation rate and calculated the methane emission to the atmosphere. Dissolved methane concentration ranged from 237 to 876 nmol L-1 during the study period. The change degree of dissolved methane concentration within a river segment ([CH4]downstream – [CH4]upstream) ranged from -113 to 143 nmol L-1. Methane oxidation rate and emission flux to the atmosphere were 1.3 ~ 8.8 nmol L-1 h-1 and 11 ~ 54 nmol L-1 h-1, respectively, and the emission flux to the atmosphere was higher than the methane oxidation rate. As a result, the net methane loading rate from the riverbed was estimated to be between -50 and 117 µmol m-2 h-1. This value is comparable to the net methane production (methane production – methane oxidation) of 100 µmol m-2 h-1 in the sediment at eutrophic Lake Kasumigaura, Japan, suggesting that the riverbed may have the same rate of internally loaded methane production as the sediment of eutrophic lakes.