Performance characteristics of an inertial electrostatic confinement fusion triple-grid system are experimentally studied to provide an ample fusion reaction rate under a lower-gas-pressure region to make the operation free from glow discharge restrictions between the discharge voltage, current, and gas pressure. With a filament to provide sufficient electrons, the operating gas pressure is found to reduce down to 1/5 for the same discharge current and voltage. Although the gas pressure region that was achieved still remains the region where the fusion reaction between the ion beam and background gas is dominant, the neutron yield normalized by the gas pressure in the triple-grid system shows higher value than the conventional single-grid system.