A method for observing a paranasal sinus, includes, inserting an endoscope into the paranasal sinus, emitting light from the outside of a body after inserting the endoscope into the paranasal sinus so that a landmark is formed on an inner wall of the paranasal sinus, and operating the endoscope toward the periphery of the landmark so that at least part of the landmark is shown in an endoscopic image.
