The present invention is directed to defrost a reducing agent with cooling water of an engine and to prevent water scale from being deposited and accumulated on a cooling water pipe for circulating the cooling water. A frozen reducing agent is defrosted by being heated with cooling water of an engine. A cooling water pipe for guiding the cooling water to a reducing agent tank is provided with a cutoff valve. To defrost the reducing agent, the cutoff valve is opened. Even in the case where the reducing agent is not frozen, the cutoff valve is opened for a predetermined period after start of the cutoff valve to circulate the cooling water in the cooling water pipe.