A method of detecting a gas concentration using a gas concentration detection apparatus comprising: a measuring gas chamber into which a measuring gas is introduced; a pump cell provided with a solid electrolyte having oxygen ion conductivity and a pair of electrodes (21, 25 or 21A, 21B) provided on the solid electrolyte; a sensor cell provided with a solid electrolyte having oxygen ion conductivity and a pair of electrodes (22, 25) disposed on the solid electrolyte; and a pump cell controller configured to adjust an oxygen concentration in the measuring gas of the measuring gas chamber by applying a regular voltage to the pump cell when detecting a concentration of a specific gas in the measuring gas; the method comprising: applying (S4, S5), by the pump cell controller, an elimination voltage in a range of 1 to 2 V for a predetermined time in a range of 5 to 30 seconds to the pump cell at a start-up point before detecting a concentration of a specific gas in the measuring gas to decompose substances in the measuring gas chamber to generate a reduced gas, which changes a value of an output current of the sensor cell from a positive value above zero to a negative value below zero, to remove oxygen occluded at a sensor electrode disposed in the measuring gas chamber, among a pair of electrodes (22, 25) of the sensor cell, and applying a regular voltage to the pump cell after the elimination voltage has been applied for the predetermined time to detect the concentration of a specific gas in the measuring gas; wherein the concentration of the specific gas in the measuring gas is detected based on an output current from the sensor cell in which oxygen occluded at the sensor electrode has been removed. a measuring gas chamber into which a measuring gas is introduced; a pump cell provided with a solid electrolyte having oxygen ion conductivity and a pair of electrodes (21, 25 or 21A, 21B) provided on the solid electrolyte; a sensor cell provided with a solid electrolyte having oxygen ion conductivity and a pair of electrodes (22, 25) disposed on the solid electrolyte; and a pump cell controller configured to adjust an oxygen concentration in the measuring gas of the measuring gas chamber by applying a regular voltage to the pump cell when detecting a concentration of a specific gas in the measuring gas; the method comprising: applying (S4, S5), by the pump cell controller, an elimination voltage in a range of 1 to 2 V for a predetermined time in a range of 5 to 30 seconds to the pump cell at a start-up point before detecting a concentration of a specific gas in the measuring gas to decompose substances in the measuring gas chamber to generate a reduced gas, which changes a value of an output current of the sensor cell from a positive value above zero to a negative value below zero, to remove oxygen occluded at a sensor electrode disposed in the measuring gas chamber, among a pair of electrodes (22, 25) of the sensor cell, and applying a regular voltage to the pump cell after the elimination voltage has been applied for the predetermined time to detect the concentration of a specific gas in the measuring gas; wherein the concentration of the specific gas in the measuring gas is detected based on an output current from the sensor cell in which oxygen occluded at the sensor electrode has been removed. applying (S4, S5), by the pump cell controller, an elimination voltage in a range of 1 to 2 V for a predetermined time in a range of 5 to 30 seconds to the pump cell at a start-up point before detecting a concentration of a specific gas in the measuring gas to decompose substances in the measuring gas chamber to generate a reduced gas, which changes a value of an output current of the sensor cell from a positive value above zero to a negative value below zero, to remove oxygen occluded at a sensor electrode disposed in the measuring gas chamber, among a pair of electrodes (22, 25) of the sensor cell, and applying a regular voltage to the pump cell after the elimination voltage has been applied for the predetermined time to detect the concentration of a specific gas in the measuring gas; wherein the concentration of the specific gas in the measuring gas is detected based on an output current from the sensor cell in which oxygen occluded at the sensor electrode has been removed.