PROBLEM TO BE SOLVED: To prevent corrosion and damages to a pipe and excel the response of nitrogen removal reaction by installing a plate for preventing dripping of urea water in the bottom face part of an evaporator and depositing the urea water on the plate. SOLUTION: In the evaporator 12, an opening part 13 formed in a side wall of an evaporator vessel 12a is fixed in an opening part 11a of a pipe 11 for a waste gas while being kept facing on the downstream side of a waste gas, so that the evaporator 12 can be installed in the pipe 11 for the waste gas. The plate 14 for preventing dripping of urea water is formed in the bottom face part of the evaporator vessel 12a, and therefore, when urea water as a reducing agent, from the evaporator vessel 12a is jetted out through the opening part 13, the urea water is not deposited on the pipe 11 but deposited on the plate 14 for preventing dripping.
