In a gas sensor determining a NOx concentration in a measurement gas based on a pump current flowing between a NOx measurement electrode and an outer pump electrode, the outer pump electrode has catalytic activity inactivated for HC and CO, so that a sensor element further includes a HC sensor part having a mixed potential cell constituted by the outer pump electrode, a reference electrode, and a solid electrolyte between these electrodes, and a HC mode for determining a HC concentration in the measurement gas based on a potential difference between the outer pump electrode and the reference electrode when the sensor element is heated to a temperature which is 400° C. or higher and 650° C. or lower and a NOx mode for determining a NOx concentration in the measurement gas based on the pump current can selectively be performed based on the temperature of the sensor element.