It is possible to obtain an endoscope-image with high image quality while keeping the diameter of a distal end portion of an endoscope small, even if the direction of the field of view of the endoscope is made adjustable. Provided is an endoscope optical system (1) including a negative lens (3) that focuses light entering along an incident optical axis (C1), a first prism (5) that deflects and emits the light focused by the negative lens (3) in a direction along a first axis (C2) substantially orthogonal to the incident optical axis (C1), a second prism (6) that has a reflecting surface, which deflects the light emitted from the first prism (5) in a direction of a second axis (C3) substantially orthogonal to the first axis (C2), and that is disposed so as to face the first prism (5), and a positive lens (8) that focuses the light emitted from the second prism (6), in this order from an object side, wherein the negative lens (3) and the first prism (5) are provided so as to be rotatable about the first axis (C2) relative to the second prism (6).
