Gas permeation mechanisms through a micropore of a vitreous silica (v-SiO2) membrane were studied using a molecular dynamics (MD) simulation. Virtual v-SiO2 membranes were prepared by the melt-quench methods using the modified Born-Mayer-Huggins pair potential and Stillinger-Waber three-body interactions. The particle-generating non-equilibrium MD technique was employed in order to simulate gas permeation phenomena, where permeating molecules were modeled as Lennard-Jones particles. This simulation method accommodates a change in the number of particles in a unit cell and, hence, an accurate simulation of the steady-state process of permeation can be achieved, The dependencies of permeance on temperature and pressure were discussed. For cylindrical pores of about 5 Angstrom in diameter, the calculated temperature dependencies of the permeance of He-like LJ particles were similar to those predicted by the normal Knudsen permeation mechanism, while, for CO2 permeation, a temperature dependency larger than He and a significant deviation from the Knudsen's could be observed. In the relatively high-temperature region (400-800 K), the simulated permeance of CO2 was nearly independent of the upstream pressure, while at the temperature below 300 K, a pressure dependency of permeance was observed. Simulations of adsorption conducted on the same unit cell yielded a Henry-type isotherm at 400 K and a Langmuir-type isotherm at 260 K. These results indicate that gas-Eke permeation occurred in the higher-temperature region, where the permeation flux is proportional to the pressure drop across the pore. However, at lower temperatures, the transports of molecules as some type of adsorption phase might be dominant in such a small pore. A simple gas permeation model, considering the effect of the pore wall potential field and Langmuir type adsorption within a micropore explained those permeation properties of CO2 well.