Abstract: A gas sensor comprises a first surface-acoustic-wave device, at least one further surface-acoustic-wave device, and a control device. The first surface-acoustic-wave device includes a piezoelectric substrate, a pair of transducers and an external circuit. The pair of transducers consists of a first transducer and a second transducer, and they are formed on two sides of the piezoelectric substrate. The first transducer is utilized to generate a surface acoustic wave on the piezoelectric substrate. The external circuit electrically connects to the pair of transducers. At least one further surface-acoustic-wave device includes at least one first surface-acoustic-device and a sensing porous thin film of which two sides are formed on the pair of the transducers. The control device is utilized to control only one external circuit to become activated at one time.