Provided is a gas sensor improving accuracy of detection of specific gas in measurement gas, while maintaining high responsiveness in detection of the specific gas. A sensor element of a gas sensor includes a first solid electrolyte and second solid electrolyte having oxygen ionic conductivity, a measurement gas chamber into which measurement gas is introduced, a first reference gas chamber and second reference gas chamber into which reference gas is introduced, a first pump cell, a second pump cell, a sensor cell, and a heater. The sensor cell detects a specific gas component in the measurement gas having an oxygen concentration adjusted by the pump cells. The heater is located opposite to a second principal surface of the second solid electrolyte on which the sensor cell is formed.