The deposition rate of tungsten selectively prepared by hydrogen reduction of WF6 was measured, and the encroachment produced by inherent silicon reduction even in the presence of H-2 gas was examined by cross-sectional TEM and SEM. In the WF6-H2 system, the degree of encroachment is not explained by the Si reduction reaction alone, but is rather related to the Si reduction time decreasing with increasing deposition rate of H-2-reduced tungsten film, because a blocking layer is formed above the Si-reduced tungsten. This results in a lesser degree of encroachment. Consequently a high deposition rate of H-2-reduced tungsten can decrease the degree of encroachment. By calculation, a thickness of 6.8-13.3 nm is necessary for H-2-reduced tungsten to prevent WF6 gas from reacting with Si.