The present invention provides a biological treatment method of biologically treating the subject water containing phenol, thiocyanate and the like, which improves the quality of treated water, while suppressing a reduction in treatment efficiency. As a means for achieving the aforementioned object, the present invention relates to a biological treatment method including introducing subject water containing a COD component, wherein the COD component is at least one of phenol and thiocyanate, into a biological treatment tank containing sludge containing bacteria capable of decomposing the COD component to thereby biologically treat the COD component with the bacteria, wherein, before the introduction of the subject water into the biological treatment tank, a step of counting the total number of bacteria contained in the sludge is carried out, so that the amount of the COD component loaded on a single bacterium per unit time can be controlled within a predetermined range.