PROBLEM TO BE SOLVED: To provide a method and an apparatus for producing a feed additive, in which animalcules can be stably cultured and obtained as raw material of the feed additive, even if raw water has heavy load or load fluctuation is large.SOLUTION: The method for producing a feed additive by a main treatment apparatus 2 comprises: a first biological treatment step of introducing organic waste water to a first biological treatment tank 10 and performing aerobic bacterial treatment by bacteria to obtain first biological treatment liquid containing dispersible bacteria； a second biological treatment step of introducing the first biological treatment liquid to a second biological treatment tank 20 and performing activated sludge treatment to obtain second biological treatment liquid； and a sludge separation step of taking out a part of in-tank sludge in the second biological treatment tank to obtain it as raw material of the feed additive； wherein at least a part of the organic waste water is supplied and subjected to treatment to a sub treatment apparatus 3 so that the load of the first biological treatment tank 10 is not greater than prescribed load.
