Method of analyzing gas using quartz oscillator and apparatus therefor

A gas analyzing apparatus including a reactor for decomposing a target substance contained in a gas to produce a product gas containing a decomposition product, a contacting chamber connected to the reactor and having a quartz oscillator disposed therewithin. The quartz oscillator has opposing surfaces each provided with an electrode, at least one of the electrodes being reactable with the decomposition product so that the decomposition product when contacted with the reactable electrode is reacted with the reactable electrode to cause a frequency deviation which is detected by a frequency measuring device.