A sensor element detects exhaust gas emitted from an internal combustion engine. A heater is supplied with electricity from a power source so as to heat the sensor element. A detection unit detects heater voltage across the heater. A degradation determination unit determines whether the heater is degraded in accordance with at least one of the heater voltage and a diagnosis value, which changes according to the heater voltage. The degradation determination unit determines whether the heater is degraded further in accordance with at least one of power-supply voltage of the power source and a driving state of an electric device, which is supplied with electricity from the power source.