[en] The origin of the H^- ion beam on-uniformity under the Cesium seeded condition was studied in the JAEA 10 Ampere negative ion source by measuring the profiles of the beam intensity and plasma parameters. The numerical analyses, such as the trace of particles (the electron, the H⁰ atom and the H^- ion) trajectories using the Monte Carlo method, were also applied to consider the experimental results. As a result, a new knowledge has been obtained as follows: 1) Under the Cesium seeded condition, a high current density beam of the H^- ion was extracted from higher electron temperature region in contrast with the pure volume condition. To explain the reasons for the difference of beam profiles, the H⁰ atoms which are primary particles of the H^- ions in the surface production process were numerically traced by using the Monte Carlo method. The result indicates that the H⁰ atom flux locally increases near the high electron temperature region in which the H⁰ atom production is enhanced, and possibly affects the H^- ion surface-production profile. As a factor contributing to the increase of the H^- ion beam intensity near the high electron temperature region, the numerical analysis using rate equation suggested that the most surface-produced H^- ions were extracted before the destruction by high energy electrons. 2) The first electrons which affect the H^- ion beam profile were numerically traced. As a consequence, the localization of the first electrons by the Bx∇B drift was reconfirmed. By changing the filament positions to suppress this electronic drift, the H^- ion beam uniformity was improved. (author)