To provide a gas measuring apparatus and method capable of detecting a very small amt. of the component gas in mixed gas.

Voltage wherein sine wave voltage and random voltage are superposed one upon another by a sine wave voltage generator 16 and a noise generator 18 is applied to a heater 12B and the conductivity of an N-type semiconductor 18 is detected as voltage by a detection resistor RL and the concn. of the component gas in mixed gas is operated by a microcomputer 20. Since weak output voltage exceeding a detection limit temporarily can be detected on the basis of the random change on the surface of an N-type semiconductor 12A, the concn. of a very small amt. of the component gas in the mixed gas can be measured.