Densities of silane and hydrogen gas mixtures used for the production of thin silicon solar cell were measured using a quartz sensor, which output depends on viscosity and molecular weight of gas measured. The measurement for the silane-hydrogen gas mixtures revealed that the quartz sensor output depends on the silane and hydrogen densitites, can derive each density in the mixtures. From the results of the measurement, working curves were dereived for the pressure of 133-1333.0 Pa for the industrial production at room temperature. Resolusions of the density measured are 0.03 vol.% at total pressure of 1333.0 Pa, and are comparable to the results by a quartz pressure gauge previously reported. Finally, it was shown that the silane density ratio does not agree with the silane flow ratio, indicating that in-situ silane density measurement is important to control silane density.