A partition member with a buffer plate disposed on the top surface thereof is disposed in a region belonging to a surface-to-be-processed of a substrate-to-be-processed held in a processing apparatus. An injector having cross-sectional areas decreasing toward the forward end thereof is disposed in the partition member. The injector has a number of injection holes formed length-wise therein at a constant pitch and in the same bore, whereby a processing gas can be injected very uniformly in the longitudinal direction and diffuse in the partition member. Then the processing gas is passed through vent holes formed in the buffer plate uniformly into the processing chamber. The processing gas can be fed uniformly onto the surface-to-be-processed of the substrate-to-be-processed, and a deposited thin film can have high intra-surface thickness uniformity.