To prevent high pressure ammonia produced from aqueous urea in an aqueous urea supplying pipe from leaking out of the aqueous urea supplying pipe when a urea water injection nozzle is clogged.

A spray nozzle 7 is provided in a supply pipe 6 protruding into an interior of an exhaust pipe 3. A branch pipe 6a of the supply pipe 6 is connected to an aqueous urea tank 12 through a non-return valve 8, a pump 11 and the like. A branch pipe 6b is connected to a water supply source for replacing aqueous urea through a non-return valve 14 and the like. A pressure receiving side of a safety valve 16 is connected to downstream sides of the two non-return valves 8, 14 connected to the branch pipes 6a, 6b and a pressure relieving side of the safety valve 16 is connected to a releasing pipe 17 guided into the exhaust pipe 3. If the injection nozzle 7 is clogged for some reason during operation of an engine 1, temperature of the aqueous urea within the supply pipe 6 is raised by high temperature exhaust gas within the exhaust pipe 3 to decompose the urea to produce high pressure ammonia gas. The safety valve 16 is activated to release the abnormally highly pressurized ammonia gas within the pipe through the releasing pipe 17 into the exhaust pipe 3.