In large negative ion sources of the neutral beam injector (NBI) for large fusion devices, such as ITER and JT-60SA, one of issues is that negative ion beam might be partially intercepted at acceleration grids due to a spatial non-uniformity of negative ion production on large extraction area. Previous experiments have shown that fast electrons emitted from filament cathodes are transported in a longitudinal direction by B × grad B drift and the spatial distribution of electron temperature (Te) is strongly related with the non-uniformity of negative ion production. In this study, a three-dimensional electron transport analysis code including detailed collision processes has been developed to clarify a physical mechanism of non-uniform Te distribution. Electron density and temperature in the analysis agree well with measurements by Langmuir probe in the JAEA 10A ion source. Then, this study has clarified that the non-uniformity of Te distribution is mainly caused by the following reasons; i) fast electrons drifted in the longitudinal direction survive near the end wall with high energy up to E = 25 - 60 eV and ii) they produce thermal electrons by collision with plasma particles there.