An insertion method includes: specifying a part of a mucous membrane which lies between a nasal cavity and a maxillary sinus and which is located in a space surrounded by an inferior nasal concha, a lacrimal bone, an ethmoid bone, and a palatine bone, as an opening formation position where an opening connected to the maxillary sinus is formed； forming the opening at the opening formation position； and inserting a medical device into the maxillary sinus through the formed opening.
