Using a 1-D particle code, we have analyzed characteristics of an Inertial Electrostatic Confinement Fusion device with external ion source which is added to enable low pressure operation. When the pressure becomes low, though neutron yield decreases, the decreasing amount is less than estimated from the decrease in background (target) gas density and it is confirmed that ions are accelerated efficiently with little energy loss through charge-exchange collision with background gas at low pressure. And when the pressure is lower than 0.05Pa, almost all injected ions reach to the cathode and it is expected that applying high geometrical transparency enhances accumulation of ion and enlarges neutron yield.