An apparatus for distinguishing a liquid reducing agent which judges empty with a higher precision than heretofore is provided. The apparatus for distinguishing a liquid reducing agent is provided with a concentration sensor and a liquid-level sensor in storage tank of liquid reducing agent, and is characterized in that empty is judged by utilizing the liquid-level based on the liquid-level sensor in addition to the concentration based on the concentration sensor (step 3 and step 5). Even if a bubble is adhering to the concentration sensor and an erroneous signal indicative of empty is output although the residual quantity of liquid reducing agent is sufficient, the liquid-level sensor does not output a signal indicative of empty. Consequently, erroneous judgment is prevented and accurate judgment can be made as compared to heretofore.