Abstract: A wetted wall type method of humidifying gas with a liquid which is composed, or mainly composed, of water wherein the liquid flows down a heated vertical wall to form a wetted wall and gas flows along the wetted wall in contact with the liquid on the wetted wall to increase the humidity contained in the gas, comprises supplying a greater amount of liquid than the amount of liquid used or evaporated for increasing the humidity, and recirculating the liquid which is not evaporated from an outlet of the wetted wall to a liquid supply portion, the amount of the flowing liquid being adapted to satisfy the following equation:q<5.99.times.10.sup.5 .GAMMA..sup.2.12 (1)wherein q is the heat flux in W/m.sup.2 K where W=watts, K=temperature in degrees Kelvin, and .GAMMA. is the amount of flowing water with regard to the mass of water per unit wet width in kg/ms.