To accurately calculate a methanol concentration when detecting the concentration by using a concentration sensor utilizing a surface acoustic wave.

The methanol concentration is detected by using the self-exciting concentration sensor 10 having a short channel 34A and an open channel 34B. On this occasion, oscillation frequencies fs and fo and gain adjusting voltages Vs and Vo are previously measured, the fs, fo, Vs, and Vo being output from closed-loop circuits 22A and 22B on the short channel side and on the open channel side, respectively. Measurement values thereof are used to set four approximate expressions for calculating respective output values, that is, fs, fo, Vs, and Vo, with methanol concentration and formic acid concentration used as coefficients, and environmental temperature used as a variable. The range of environmental temperature is limited by using the measurement value of the oscillation frequency fs giving low sensitivity to the methanol concentration and to the acid concentration but sensitively reacting to the environmental temperature. Then, solutions of the environmental temperature are found from the respective approximate expressions to acquire the methanol concentration from a calculation result thereof.

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