In the atmospheric pressure plasma CVD system, improvement of the process environment by utilizing the reactivity between SiH₄ and H₂0 was examined. The cleaning gas containing He and SiH₄ was circulated by the gas circulation system in heated condition, and H₂O which remained in the process atmosphere was measured by APIMS (Atmospheric Pressure Ionization Mass Spectrometer). Hydrogenated amorphous silicon (a-Si:H) films were deposited by the atmospheric pressure plasma CVD before and after the SiH₄ circulation cleaning. In order to evaluate the effectiveness of the SiH₄ circulation cleaning, contaminations such as O, N and C were analyzed by SIMS (Secondary Ion Mass Spectroscopy). Furthermore, the photoconductivity and the dark conductivity were compared. The results showed that the H₂0 concentration in the process atmosphere drastically decreased by the SiH₄ circulation, and the O content in the a-Si:H film decreased similarly. The O content in the film after the SiH₄ circulation cleaning was 5×10¹⁸ /cm³, which value was as same as or less than that by the conventional plasma CVD technique. The dark conductivity decreased, and the electrical property was improved. It was because the O content in the film decreased by the SiH₄ circulation cleaning.