To enable a simple structure of an endoscope to improve its observation visual field by removing a fouling which is attached to an observation window.

In a distal end structure of an endoscope where a pointed head aperture 3 of a treatment tool insertion channel in which a treatment tool 10 is inserted and the observation window 2 are disposed side by side, the side of the distal end aperture 3 which is near the observation window 2 is expanded to approach the observation window 2 and to form an expanded aperture section 3A, and suction is performed through the expanded aperture section 3A to remove the fouling on the observation window 2.