If the concentration measured by a sensor is below a predetermined threshold, the liquid is judged to be a different type of aqueous solution, and the judgment that the liquid is different is verified. If the different type judgment is valid, the number of judgments of different type is counted according to a predetermined count. If the number of judgments is a determinated threshold or more, the different type judgment is determined. At least one of the predetermined count and the determined threshold is dynamically set according to the liquid level in a reducing agent tank. As a result, count or/and different type judgment determination are carried out in consideration of the convection varying with the level change, and thus it is accurately judged whether or not the liquid is a different aqueous solution.