A substrate processing apparatus has a simple structure in which the number of nozzles is reduced. In a mixer part, photoresist which is supplied from a photoresist supply through a lower arm portion of the nozzle arm is mixed with solvent which is supplied from a solvent supply through a pipe. Following this mixing, the photoresist and the solvent flow through an upper arm portion and are ejected from a nozzle toward a substrate. In the nozzle arm, a viscometer is disposed to the upper arm portion. The viscometer measures a viscosity of photoresist solution which is obtained by mixing in the mixer part. In accordance with the measurement from the viscometer, the quantity of solvent supplied from the solvent supply is controlled, thereby obtaining a photoresist solution having a desired viscosity.