A biological treatment method and device for organic wastewater, whereby the amount of minute organisms which reduce the amount of excess sludge and held within an activated sludge tank can be stabilized so that the treatment can be stabilized, are provided. Substrates for growing dispersed bacteria and/or dispersed bacteria are supplied to a sludge treatment tank 51 which reduces the sludge by the minute organisms, so that the minute organisms grow stably and reduce the sludge. Specifically, a first and a second biological treatment step are conducted respectively in a first activated sludge tank 21 and a second biological treatment tank 31. The sludge included in effluent which flows from the biological steps is separated in a sedimentation tank 41. The generated sludge obtained is supplied to the sludge treatment tank 51. The water to be treated which is bypassed via a bypass path 26 is introduced to a sludge treatment tank 51 as substrates to be assimilated by bacteria so that dispersed bacteria are made to grow in the sludge treatment tank 51 and the minute organisms are held stably in the sludge treatment tank 51 to promote the reduction of sludge.