To structure a concentration sensor to be compact first and then to allow concentrations to be detected accurately, as to a concentration sensor structured to detect the concentrations of chemical substances in a tested solution by using a surface acoustic wave.

This concentration sensor is structured as a self-exciting concentration sensor having a short channel 34A and an open channel 34B. On that basis, operation is stopped of a high-frequency amplifier 42B of a circuit 40B for oscillation connected to the open channel 34B when a high-frequency amplifier 42A is in operation of a circuit 40A for oscillation connected to the short channel 34A. Further, operation of the amplifier 42A is stopped when the high-frequency amplifier 42B is in operation. This prevents interference from occurring between self-oscillation in a closed-loop circuit 22A on the short channel side and self-oscillation in a closed-loop circuit 22B on the open channel side.

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