A NOx reduction catalyst and an ammonia slip oxidation catalyst are disposed in an exhaust system in this order, and also, an electric fan is disposed on piping which communicates an upper space of a storage tank storing therein a reducing agent with the exhaust upstream of the NOx reduction catalyst. Then, when the temperature of the ammonia slip oxidation catalyst reaches or exceeds the temperature for activating a catalyst thereof, the electric fan is operated for a predetermined period of time, so that the gas (ammonia series gas) in the upper space of the storage tank is forcibly discharged to the upstream side of the NOx reduction catalyst. Further, a discharge-forcing device, such as an electric fan or the like, forcibly discharging the gas in the upper space of the storage tank, an adsorbing device temporarily adsorbing thereto the forcibly discharged gas and an oxidation catalyst oxidizing the gas desorbed from the adsorbing device, may be disposed to the storage tank in this order.