A method and apparatus of anaerobic treatment which makes it possible to prevent disintegration of granular sludge and perform high-load high-velocity anaerobic treatment stably even in the case where treatment is performed under conditions where the granular sludge is easily disintegrated. In a reaction tank 20 which retains the granular sludge, nitric acid or nitrous acid is adjusted so that they have a certain concentration range. For example, nitric acid is added to the liquid to be treated by connecting the nitric acid addition path 12 to a midway point of the path of the liquid to be treated 31 which introduces the liquid to be treated to the reaction tank 20, and nitric acid is mixed with the liquid to be treated. By introducing nitric acids within the reaction tank 20, denitrifying bacteria etc. can multiply, and disintegration of the granules is prevented.