Substrate processing method and substrate processing apparatus

A substrate treatment method, whereby it becomes possible to supply a silicon-containing aqueous phosphate solution to a substrate having both of a silicon oxide film and a silicon nitride film exposed on the surface thereof so as to selectively etch the silicon nitride film. The method includes: a step of storing an aqueous phosphate solution containing silicon at a silicon concentration falling within a specified range in a tank; a step of supplying the aqueous phosphate solution in the tank to a nozzle and supplying the aqueous phosphate solution to the substrate through the nozzle to treat the substrate; a collection step of collecting the aqueous phosphate solution used for the treatment of the substrate in the tank; a first aqueous phosphate solution supply step of supplying a first aqueous phosphate solution containing silicon at a first concentration to the tank; a second aqueous phosphate solution supply step of supplying a second aqueous phosphate solution containing silicon at a second concentratio