Abstract: A substrate processing device, which processes by immersing the substrate in the processing liquid comprising a mixture of a chemical and a diluting liquid, is provided with: a processing tank (1) that retains the processing liquid; heating means (2, 3) that heat the processing liquid; a temperature detection means (4) that detects the temperature of the processing liquid; a temperature control means (5) that operates the aforementioned heating means (2, 3) in a manner so that the detected temperature approaches a set temperature; a replenishing means (6) that replenishes the diluting liquid in the processing liquid; a concentration detection means (7) that detects the concentration of the processing liquid by measuring the light absorption characteristics of the processing liquid; and a concentration control means (8) that operates the aforementioned replenishing means (6) in a manner so that the detected concentration approaches a set concentration.