To prevent a processing gas from leaking from a processing tank for processing a surface of a substrate and to stabilize flow of the processing gas in a processing space.A substrate 9 is conveyed into the inside of a processing tank 10 through an entrance port 13 by a conveyor 20 and positioned in a processing space 19. A processing gas is supplied to the processing space 19 by a supply system 30, and the substrate 9 is surface processed. Subsequently, the substrate 9 is conveyed out through an exit port 14. Gas inside of the processing tank 10 is exhausted by an exhaust system 40. The exhausting of gas causes gas outside of the processing tank 10 to inflow into the inside of the processing tank 10 through the openings 13, 14 such that an average flow velocity of the inflow gas is at least 0.1 m/sec yet smaller than a velocity that would allow the inflow gas to reach the processing space 19.