A method for detecting a substance in a liquid with high accuracy and high sensitivity includes recording frequency drifts of output frequencies of oscillator circuit outputs of m sensing oscillator circuits respectively including sensing SAW elements and frequency drifts of output frequencies of oscillator circuit outputs of n reference oscillator circuits respectively including reference SAW elements; determining at least one of the sensing oscillator circuits whose frequency drifts fall within a predetermined range and at least one of the reference oscillator circuits whose frequency drifts fall within the predetermined range to be acceptable; selecting at least one of the sensing oscillator circuits from among the acceptable sensing oscillator circuits and at least one of the reference oscillator circuits from among the acceptable reference oscillator circuits; and comparing an oscillator circuit output of the selected at least one of sensing oscillator circuits with an oscillator circuit output of the selected at least one of reference oscillator circuits to thereby detect a detection target substance.