Provided are a method for treating wastewater and an activator for treating wastewater with which it is possible to maximize the utilization of microorganisms such as Bacillus bacteria in an activated sludge treatment of wastewater while minimizing treatment cost. A method for treating wastewater in which wastewater including organic matter is introduced into a treatment tank and the wastewater is subjected to an activated sludge treatment by microorganisms in the treatment tank, wherein an activator containing a component for activating the microorganisms is added to the wastewater to be subjected to the activated sludge treatment in the treatment tank; and at least 50% (by quantity) of the entirety of the activator has a particle size of less than 10 µm. Also provided is an activator therefor. The microorganisms preferably include Bacillus bacteria.