To provide an organic sludge treatment method capable of especially lowering water content of a dehydrated cake of sludge as a result of investigation of a polymeric flocculant adding method for the purpose of developing a recipe capable of developing high effect only by devising an addition method.

A cationic polymeric flocculant containing at least one kind of a specific cationic structural unit or an amphoteric polymeric flocculant containing at least one kind of a specific cationic structural unit and at least one kind of a specific anionic structural unit is added to and mixed with organic sludge. In this case, adding operation and mixing operation of the flocculant are respectively repeated one or more times, preferably, twice in the same amount to dehydrate organic sludge by a dehydrator.