A concentration sensor for detecting a concentration of a liquid reducing agent through heat transfer characteristics between separate two points is provided in a storage tank for storing a liquid supplied to an NOx reduction catalyst. When the concentration read from the concentration sensor is 0% or less, the liquid is discriminated to be water, that is, it is utilized that when water is converted to the concentration of a liquid reducing agent, it is 0%, and it is also 0% even when the concentration is reduced by convection. When the concentration is greater than a predetermined value, it is discriminated that no liquid is present, since it is considered that the output of the sensor is abnormal because of the absence of a liquid in the tank. On the other hand, when the concentration is greater than 0% and less than a predetermined value, the liquid is discriminated to be the liquid reducing agent, provided that the concentration value may include an error.