Impulse ratio is important factor for dielectric design of power equipment. Concerning vacuum insulation, simple comparison between average voltage of AC withstands voltage test and that of impulse voltage test sometimes result in wrong impulse ratio, because discharge current is different for each test circuit. The difference affects the measurement of average breakdown voltage due to the de-conditioning effect of vacuum insulation. In this paper, discharge current of AC and impulse voltage circuit was designed so that negative effect of de-conditioning was suppressed. In addition, dependence of impulse ratio on insulation gap length was examined.