A substrate processing apparatus includes a spin chuck that holds a substrate, and a fluid nozzle disposed to face a principal surface of the substrate which is held by the spin chuck. The fluid nozzle includes a gas discharge port from which a gas is discharged radially from the center side of the principal surface of the substrate to the peripheral edge side, and a gas flow passage through which the gas is supplied to the gas discharge port, the gas flow passage having a tubular shape along an intersecting direction with respect to the principal surface of the substrate. The gas flow passage has a gas retaining portion whose flow passage cross-sectional area is larger than other portions of the gas flow passage, and a rectifying structure provided in a portion of the gas flow passage different from the gas retaining portion, the rectifying structure that rectifies a flow of the gas in the gas flow passage.