This optical adapter for an endoscope includes a plurality of incident optical systems eccentrically disposed with each other, an optical axis of each of the plurality of incident optical systems being spaced away from an axis of the optical adapter for an endoscope with an interval； a brightness stop including a plurality of apertures corresponding to the plurality of incident optical systems respectively； a relay lens configured to relay incident light entering each of the plurality of incident optical systems to the corresponding aperture； and a distal light-shielding portion which is disposed more distally than the relay lens, wherein the plurality of incident optical systems, the relay lens, and the brightness stop are disposed in this order from a distal end side toward a proximal end side of the optical adapter for an endoscope along an incident direction of the incident light.
