A rationality diagnosis method and a rationality diagnosis apparatus for a sensor inside a tank are provided. The rationality diagnosis method and the rationality diagnosis apparatus can accurately and easily perform a diagnosis to determine whether rationality of all sensors is maintained.The rationality diagnosis method for the sensor inside the tank performs a rationality diagnosis of a concentration sensor, a liquid level sensor and a temperature sensor that are provided inside the tank that stores liquid. The rationality diagnosis method is characterized by diagnosing whether the rationality is lost in one of the concentration sensor, the liquid level sensor and the temperature sensor by determining whether there exists any contradiction between respective sensor values of the concentration sensor, the liquid level sensor and the temperature sensor.