An inertial electrostatic confinement (IEC) fusion device driven by a ring-shaped built-in ion source is proposed and designed aiming at a reduced operating gas pressure in order to explore a possibility of a drastic enhancement in the fusion reaction rate in the envisaged beam-beam collision regime. In the present scheme ions will be extracted from a ring-shaped magnetron discharge plasma toward an IEC cathode grid placed concentrically at the center. A prototype ion source showed an accessible pressure of 5 mPa, which is hundreds times as low as the conventional glow-discharge-driven IEC. Dependence of the ion source current and extraction efficiency on the central IEC cathode voltage was studied by prototype experiments and numerical calculations. An IEC device with a built-in ion source was then designed based on these results. The expected IEC grid current is ~0.4 mA at 5 mPa, where observation of the beam-beam fusion contribution is anticipated.