According to the method, a vertical tube (1) is arranged in a liquid contained in a tank, and the liquid is made to flow through the tube from top to bottom by means of a liquid transport device placed in the tube and provided with an impeller (4), and gas is conducted into the liquid via a gas feeding device (6) placed after the impeller, when seen in the direction of the liquid flow. The turbulent motion of the liquid, caused by the impeller, is attenuated so that in the tube, after the impeller and in the vicinity thereof, there are fitted baffles (5) in a radial fashion and mainly parallel to the tube. Gas is fed into the liquid, at the earliest stage with respect to the liquid flow direction after the turbulent motion is attenuated and the liquid flows mainly in the tube direction. The impeller (4) and the baffles (5) are matched so that the direction of the liquid flow leaving the impeller blade is roughly parallel to the tangent of the intake edge of the baffle.