Provided are an endoscope optical system, an endoscope, and an endoscope system, whereby an object positioned near the outer circumferential surface of a tip optical system or an object in contact with the outer circumferential surface of a tip optical system can be clearly observed. The endoscope optical system 1 has: a tip member 2 connected to a holding member 7； and an image-forming optical system 3 arranged in the holding member 7. The tip member 2 has a tip optical system 4. The tip optical system 4 has an inner circumferential surface 4b and an outer circumferential surface 4a. One end of the inner circumferential surface 4b and one end of the outer circumferential surface 4a are positioned upon the optical axis of the image-forming optical system 3. The position of one end of the inner circumferential surface 4b is closer to the image-forming optical system 3 than the position of one end of the outer circumferential surface 4a. The area between the inner circumferential surface 4b and the outer circumferential surface 4a is filled with a transparent substance having a refractive index of greater than 1 and, as a result of being connected, a space 5 filled with gas is formed between the inner circumferential surface 4b and the image-forming optical system 3. The image-forming optical system 3 is characterized by comprising only a transmissive surface, having a field curvature aberration, and fulfilling conditional formula (1). –10 <； P' <； –0.8 (1)L'invention concerne un système optique d'endoscope, un endoscope et un système d'endoscope, grâce auxquels un objet positionné à proximité de la surface circonférentielle externe d'un système optique d'extrémité ou un objet en contact avec la surface circonférentielle externe d'un système optique d'extrémité peut être clairement observé. Le système optique d'endoscope (1) comprend : un élément d'extrémité (2) relié à un élément de support (7) ； et un système optique de formation d'image (3) disposé dans l'éléme
