A treatment method includes: inserting a treatment member into a maxillary sinus through a first opening in a nasal cavity leading to the maxillary sinus； inserting an endoscope into the maxillary sinus through a second opening leading to the maxillary sinus and different from the first opening； and performing a treatment on a treatment target site in the maxillary sinus while the treatment member and the treatment target site exist within a visual field of the endoscope.
