A metal oxide semiconductor element is arranged in a measuring chamber in which an atmosphere is introduced. A resistance of the metal oxide semiconductor element is varied corresponding to a concentration of a specific gas component. An oxygen pump, having a solid-electrolyte element with an oxygen ion transmitting property and a pair of electrodes arranged on both sides of the solid-electrolyte element, is provided. One electrode of the oxygen pump is arranged in the measuring chamber. The measuring chamber is communicated with the atmosphere through a hole as a gas diffusion resistant means. A resistance of the metal oxide semiconductor element is measured when the metal oxide semiconductor element is exposed in the atmosphere so as to obtain a concentration of a specific gas component, while an oxygen partial pressure in the measuring chamber is controlled by the oxygen pump.