Abstract: A biological removal method of phosphorus and nitrogen includes supplying a mixture gas into a gas holder. Inflow water and return water are supplied into the gas holder to make the mixture gas dissolved in the inflow water and the return water. The return water was provided from a granulation biological reaction tank. The inflow water and the return water including the dissolved mixture gas are transported to the granulation biological reaction tank. A nitrogen-based component is removed from the inflow water and the return water using methan-oxidizing bacteria granulated in the granulation biological reaction tank. Treated water without the nitrogen-based component is transported to an anoxic tank for treatment, and discharged from the anoxic tank.