The Monte Carlo method, which has been used for simulation of steady-state molecular flow and estimation of static pressure in relation to vacuum science, is applied to a calculation of the time dependent pressure distribution in a vacuum system. The calculation follows the usual simulation process, but the locations of the test particles are recorded at various values of time. In this way the pressure distribution in a vacuum system can be obtained at any time from the beginning of gas desorption. The pressure propagation along a beam line and the steady-state pressure distribution in a vacuum system with an orifice and a pump are presented as two examples.