Abstract: Provided is a substrate processing apparatus capable of supplying pure water, the nitrogen gas concentration of which is stabilized. A pure water supply path is branched such that a dissolving part generates a high concentration pure water by dissolving nitrogen gas in pure water on one path, and a degassing part generates a low concentration pure water by degassing nitrogen gas from pure water on the other path. Pure water having a desired nitrogen gas concentration C can be supplied stably by sequentially measuring the nitrogen gas concentrations C1 and C2 on their respective paths, and mixing the high concentration pure water and low concentration pure water while the opening and closing of first and second pure water valves are adjusted to control first and second flows X1 and X2, so as to satisfy the following relationships: C1·X1+C2·X2=C·V; and X1+X2=V, wherein V is a total flow of pure water.