PURPOSE: To enhance the reaction efficiency by partitioning a reaction tank by a flow dividing pipe arranged in a duct of an inlet side thereof as well as having a leading end opened into an enlarged pipe to divide a gas stream.

CONSTITUTION: To a reaction tank communicated with a reduced diameter pipe and an enlarged diameter pipe for reacting a gas-liquid or a gas-slurry mixture, a flow dividing arranged pipe 5 arranged to a part of an inlet side duct 2 as well as having a leading end opened into a center of a bottom portion of an enlarged diameter pipe 3 through a reduced diameter pipe 1 is provided. Thereby, a waste gas A is divided by this flow dividing arranged pipe 5 and a part thereof is flowed into the reduced diameter pipe directly to maintain a revolving stream and the rest is flowed into the flow dividing arranged pipe 5 and injected into a central portion of the enlarged diameter pipe as a piston stream from the leading end thereof. As described above, by using the revolving and the piston stream together, the reaction efficiency is enhanced.