Provided is a gas sensor having simpler configuration than a conventional multi-gas sensor, and being capable of measuring NOx and NH3 simultaneously. In the 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 NH3 so that a sensor element further includes a NH3 sensor part having a mixed potential cell constituted by the outer pump electrode, a reference electrode, and a solid electrolyte between these electrodes, and determination of a NH3 concentration based on a potential difference occurring between the outer pump electrode and the reference electrode and determination of a NOx concentration based on the pump current and the NH3 concentration can be performed simultaneously or selectively when the sensor element is heated to 400° C. or higher and 600° C. or lower.