Many experimental results were reported of CVD diamond synthesis with so called flow systems, which need introduction of source gas and evacuation of reactant gas. For few years, we had been concentrated of development completely closed diamond synthesizing system aimed for microgravity experiments. Recently we started the new method with graphite as a carbon source. With this method, completely closed system is successfully performed. Graphite rods are used as solid carbon source and at the same time as thermal energy source. Silicon (100) wafer was used for substrates. Hydrogen gas was introduced into the reaction chamber with the suitable initial pressure. and controlled with Joule heating. The deposits were identified with SEM and Raman spectrometry. We tried to synthesize diamond on Si substrate under the conditions of forward bias where the substrate is held at a positive potential relative to the graphite rod. And also we tried the experiment under the reverse bias.