This paper describes the initial result of simulation study of neutron production by NBI heating in the GAMMA 10 tandem mirror by using a buildup numerical simulation code. In the GAMMA 10 central-cell, high energy ions have localized in the neutral beam injection port and the other bounce point due to the perpendicular injection. By considering this mechanism, the position of the test zone for neutron irradiation is assumed and the amount of the neutron flux is evaluated. The neutron flux is about 1.3 kW/m2, for high-density mode and about 1.2 kW/m2, for hot-ion mode.