[en] Conductances of accelerator grids for hydrogen gas have been obtained by measuring the pressure distribution in high power ion source for Neutral Beam Injector. When the ion source is not operating, the conductances are in good agreement with calculated values if one considers the streaming effect which occurs in the case the distance between the grids is relatively short. When the ion source is operating, the amount of gas corresponding to ion beam is transported rapidly. The conductance increases by about 10% because arc discharge increases the gas temperature. The results obtained are applied to the design work of the ion source for JT-60 NBI and the optimum gas flow rate is estimated. (author)