Abstract: Disclosed is an anaerobic fluidized bed which comprises a fluidized bed type reaction tank, a bed of fluidized carries in the reaction tank, and means for recycling a part of treated water from the upper portion of the reaction tank to the lower portion of the same. The reaction tank has raw wastewater introduction means at the lower portion, treated water removing means and produced gas exhaust means at the upper portion. The bed is formed by filling in the reaction tank with granular carriers not larger than 300 .mu.m in diameter and not smaller than 1.1 in specific gravity, and is established so that the ratio h/H of the initial bed height h to the reaction tank effective height H is within a range of from 0.05 to 0.3.