To accurately discriminate whether liquid reducer or precursor (hereafter liquid) in a reducer tank is different kind of water solution or not using a sensor measuring concentration of the liquid from temperature rise characteristics of a heating element.

It is discriminated that the liquid is different kind of water solution when concentration measured by the sensor is less than a different kind discrimination threshold. When different kind discrimination that the liquid is the different kind of water solution is given, it is discriminated whether the different kind discrimination is appropriate or not and the number of times of different kind discrimination is counted based on predetermined count value only when the different kind discrimination is appropriate. When the number of times of different kind discrimination exceeds a settlement threshold (S51), the different kind discrimination is settled and a different kind discrimination settlement signal is output (S53). Discrimination accuracy in a temperature range where the sensor is not calibrated is improved by dynamically setting at least either one of the different kind discrimination threshold, the predetermined count value and the settlement threshold based on the temperature of the liquid in the reducer tank.