Distribution function of the hitting number per molecule which comes from a localized outgassing source, when molecule exits the long tube of lenghth L, were calculated by Monte Carlo simulation method. Results are compared with the Green's function for the one-dimensional diffusion equation of virtually extended finite region of L+4 λ/3 with Dirichlet boundary condition (values are zero at both boundaries), where λ is mean free path of the molecules. It is found, that Green's function represents that distribution function well and also is available to estimate average arriving time of molecules from a localized outgassing source at exit.