Method and apparatus for biologically oxidizing organic matter. Finely powdered organic matter is suplied into the top end of an upright, high reaction vessel (1). The vessel (1) has been filled up substantially completely by small balls, on the surfaces of which the matter to be oxidized is present in thin layers. By means of a conveyor (6) outside the vessel, the balls are transferred uppwards, whereby the balls in the reaction vessel (1) flow downwrds. Oxidizing air is blown uppwrds against the flow of balls, whereby in the upper section of the vessel, drying takes place in the first instance and then is formed an oxidizing zone automatically by microbial activity in a section, wherein the best oxidizing conditions are prevailing. Exhaust air is cleaned by means of ionic cleaners (14) and singlet oxygen generators (27) and oxidized matter is recovered from the bottom end of the reaction vessel.