A simulation method to obtain the pressure distribution in a complex vacuum system has been proposed. The method introduces a new concept with regard to the function of each component of the vacuum system. The vacuum pump is regarded as a ‘‘vacuum resistor’’ connected at one side to the perfect vacuum. The gas source is regarded as a pressure generator connected to the vacuum. The conducting pipe is regarded as a vacuum resistor between the above elements. The vacuum sides of the pump element and the gas source element are assumed to be connected together by an imaginary route, and thus the vacuum system can be regarded as a closed vacuum circuit network. Such a vacuum circuit network may be replaced by an electric circuit network of a simulator for the vacuum system. The simulator was employed for the high vacuum system of an electron microscope in order to obtain the pressure distribution. The results obtained were in good agreement with the actually measured pressure distribution.