PURPOSE: To increase the contact efficiency of gas and liquid by constituting an ejector or an ejector nozzle for spouting reaction liquid, a throat for mixing gas and liquid and a diffuser for forming the reaction liquid wherein gas is mixed with liquid into a falling film on the internal wall of a pipe and installing a plurality of ejectors in correspondence with the number of the pipe for heat exchange.

CONSTITUTION: Reaction liquid is spouted through the respective ejector nozzles 41 while keeping flow velocity. Gas contributing to reaction in the case of passing through throats 42 is sucked and caught from a gas introduction port 3. First contact of gas and liquid is performed in the parts of the throats 42. Then the reaction liquid wherein gas is mixed with liquid is blown against the internal walls of pipes 51 for heat exchange by diffusers 43 and allowed to flow down along the internal walls thereof. While reaction liquid flows down in the internal walls of the pipes 51 for heat exchange, furthermore a second contact of gas liquid and heat exchange are simultaneously performed. In other words, contact action is promoted by contact of gas and liquid in two stages and the mass-transfer rate of gas contributing to reaction into liquid is increased and velocity of dissolution of gas into liquid is increased. Therefor reaction velocity is made rapid because the mass-transfer rate of gas does not become rate of reaction.