Apparatus for treatment of wet organic matter to produce biogas, comprising a closed reactor (11) for anaerobic digestion of the wet organic matter. The anaerobic reactor comprises two vertical 5 tubes, a vertically arranged outer tube (14) defining a first reactor chamber (111) enveloping a vertically arranged inner tube (15) which is divided into a first region (112a) and a second region (112b) of a second reactor chamber (112) by a vertical partitioning wall (16). The first reactor chamber comprises a particle retaining unit (31) connecting the first and the second reactor chambers. The anaerobic reactor (11) exhibits a top discharge pipe (18) for gas developed in either 0 of the two reactor chambers (111, 112). A method for treatment of wet organic matter is also contemplated.