The optimum pressure of gas in the IREB transported chamber under an externally applied magnetic guide field is studied theoretically and experimentally in this paper. Ionization by electron avalanching and by beam electrons impact and recombination is included in the calculation of plasma density and plasma current build up. The calculations demonstrate that the optimum gas pressure increases as peak beam current, and it decreases as the risetime of beam pulse increases. The optimum nitrogen pressure is about 332Pa, when the peak beam current is 500kA, the peak voltage is 1.0MV, and the risetime of beam pulse is 60ns, showing that the net current and the time average of electric field are minimum when the gas pressure is about 332Pa, the window of nitrogen pressure for the beam transport