The cascade static lens gauge (CSLG) is an ionization vacuum gauge which has a very high sensitivity by oscillating electrons in the electrostatic lens system. The high sensitivity was not necessarily obtained experimentally in the condition of electric potential of electrodes in which electrons oscillate many times by the computer simulation study of electron trajectories without space charge. In this time electron trajectories have been simulated under the consideration of space charge. The space charge distribution is estimated to be proportional to the accumulated resident time of electrons in each mesh. The simulation method is proposed and typical results are shown.