A quartz low-pressure flow type reactor system was designed and constructed for studying catalytic reactions using a small thin film catalyst. In order to carry out the reaction more easily, the pressure around the catalyst was held at about 10 Pa. Experiments have proved the effectiveness of the reactor for in situ studies of catalysis over the films, Decomposition of nitrous oxide was adopted as a model reaction. Study of silver-cobalt oxide combined catalysts have revealed that the existence of silver-cobalt oxide interface on the surface is essential for the reaction. The surface structure and the activity were strongly affected by the preparation methods of these films.