Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo method with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Numerical results are in reasonable agreement with the profile of ~10 keV neutral atom fluxes recently measured under a geomagnetically quiet condition. Through collisional interactions, ENAs can be converted to ions and then the ionized ENAs can be reneutralized and repeat this cycle. Therefore the precipitating ENAs lose their original directionality because of gyromotions of the ionized ENAs. However, the source information is still seen in ENA data measured at low altitudes where collisions are significant, because their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a secondary source that cannot be neglected in ENA data measured in the magnetosphere.