It is possible to reduce the diameter of a tip portion of an endoscope and to acquire an endoscope image with good image quality while allowing the viewing direction of the endoscope to be changed. Provided is an endoscope optical system including a first prism (11) that deflects light incident along an incident optical axis in a direction along a first axis (A) perpendicular to the incident optical axis and emits the light a second prism (12) having two reflecting faces (12b) that fold back the light emitted from the first prism (11) along a second axis (B) separated from the first axis (A) by a gap and a third prism (13) that deflects the light folded back by the second prism (12) in a direction perpendicular to the first axis (A) and along a plane including the incident optical axis, wherein the first prism (11) is provided so as to be capable of swiveling about the first axis (A) relative to the second prism (12), and the two reflecting faces (12b) of the second prism (12) are disposed to form an obtuse angle therebetween.
