When an evaporator condensate stream from a kraft pulp mill was treated by anaerobic methane fermentation, the stream could not ferment continuously. Therefore, after the raw evaporator condensate was treated by various procedures, the treated samples were tested in a batch fermentation system. There seemed to be an inhibitor, an oily material, and the evaporator condensate could be fermented easily, after removal of the oily material, in a bio-reactor packed with pumice stone. In continuous operation, the minimum hydraulic retention time was 0.64 days, the maximum volumetric BOD removal rate was 11.47 kg/m3/d, and the maximum gas production rate for the theoretical volume was 97%. The efficiency of a bio-reactor using pumice stone was studied. The pumice stone has a large number of connected pores. The major pore sizes are 20–100 μm. The cell density of the pumice stone used in the bio-reactor was 18–19 g per liter.