A substrate processing apparatus of the present invention dilutes a sample gas by mixing the sample gas with nitrogen gas and measures the concentration of IPA gas contained in the diluted sample gas in a concentration measuring part. Then, the substrate processing apparatus calculates the concentration (C0) of IPA gas contained in the sample gas before being diluted by multiplying the measured concentration value (C1) by the reciprocal (1/P) of a dilution rate which is obtained on the basis of the amount of flow of the sample gas and that of the diluent gas, as C0=C1×(1/P). Therefore, even if IPA gas of high concentration is used, it is possible to accurately measure the concentration of the IPA gas.