A collimated beam (23) of a surface acoustic wave propagates on a piezoelectric substrate (22) while passing through sensitive film (25) to adsorb a sensing gas. Signal processing unit (40) transmits an exciting burst signal to sensor electrode (24) to excite the collimated beam (23), receives first and second returned burst signals after the collimated beam (23) has propagated, and calculates a target gas parameter by a target leakage factor of the background gas and a relation between reference gas parameters and reference leakage factors of reference gases, the leakage factor is provided by first and second attenuations of the first and second returned burst signals, respectively, using waveform data of the first and second returned burst signals.