We investigated the tolerance to reduction of transparent conducting oxide (TCO) by atomic hydrogen (H), generated on a heated catalyzer, for hydrogenated amorphous silicon (a-Si:H) solar cells by the catalytic chemical vapor deposition (Cat-CVD) method. TCO films such as SnO2 (Asahi-U) and SnO2 coated with ZnO were exposed to atomic H at various substrate holder temperatures (T-sh). It was found that a decrease in the transmittance due to reduction by atomic H for SnO2 coated with ZnO is scarcely observed, although that for SnO2 is observed strongly depending on T-sh. The reason for a decrease in the transmittance is the appearance of metallic tin (Sn) on the surface. It was also found, from the deposition of a-Si:H on SnO2, that no decrease in the transmittance occurs after deposition of a-Si:H with 100 Angstrom, showing that no damage to SnO2 occurs by penetration of atomic H through the 100-Angstrom thick a-Si:H layer. It is concluded that a-SM layers play a role of passivator for TCO, because the TCO is immediately covered with a-Si:H, with a deposition rate faster than 10 Angstrom /s, by Cat-CVD.