To provide a method for efficiently and easily removing nitrogen components from ammonia nitrogen-containing wastewater.

The method for treating wastewater containing organic matter and ammonia nitrogen comprises a process for introducing the wastewater into an anaerobic denitrification tank to obtain denitrified water, a process for introducing a part of the denitrified water into a complete nitritification tank to obtain water treated to nitritification, a process for introducing a part of the water treated to nitritification into an anammox tank to obtain water treated with anammox, and a process for separating the water treated with anammox in a membrane separator to obtain permeate and concentrate. This method further comprises a process for returning the concentrate to the anaerobic denitrification tank.