A substrate processing apparatus that performs processing by immersing a substrate into a processing liquid obtained by mixing phosphoric acid with a diluent includes a concentration sensing means for sensing the concentration of the processing liquid by measuring the absorbance characteristics of the processing liquid. The concentration sensing means includes a light-transmitting section that introduces the processing liquid into the inside to let the processing liquid pass therethrough, a light-emitting section that radiates light having a predetermined wavelength to the light-transmitting section, a light-receiving section that receives the light therefrom via the light-transmitting section, a first lens that condenses the light emitted from the light-emitting section to the light-transmitting section, a second lens that condenses the light that has passed through the light-transmitting section to the light-receiving section, and a cooling mechanism that cools at least one of these.