Trajectories of H− ions are calculated numerically by solving the 3D motion equation, including effects of collisional destruction, elastic collisions and charge exchange collisions. According to these trajectories, the extraction probability of H− ions produced at any location inside the source and the energy relaxation of extracted H− ions are estimated as a function of gas pressure. Effects of production zone and field intensity of the magnetic filter on extraction probability are also discussed. The extraction probability of surface produced H− ions is much higher than that of volume produced H− ions. The kinetic energy of extracted H− ions is reduced mainly by charge exchange collisions with H in higher pressure region and by elastic collisions with H+ in lower pressure region.We also briefly discuss the pressure dependence of extracted negative ion currents combining the present numerical results and the results of the model calculation with the zero-dimensional code.