  We design a GeV gamma ray detector based on an electron-positron pair
measurement by using the BESS-polar magnet and study it with the GEANT4
simulation code. We consider an additional counter to select an
electron-positron pair for which the energy loss by bremsstrahlung is less than
100 MeV against AMS. We obtain a significantly low detector efficiency of 3%.
However, an energy resolution better than 1% at 10-100 GeV is achieved by
employing a parameter that enables us to obtain a line sensitivity that is
almost indetical to that of the GLAST mission. The good energy resolution makes
it possible to obtain the three-dimensional map of the neutralino dark matter
annihilation line.
