An endoscope optical system includes a negative lens that focuses light entering along an incident optical axis, a first prism that deflects and emits the light from the negative lens along a first axis substantially orthogonal to the incident optical axis, a second prism having a first reflecting surface, which deflects the light from the first prism along a second axis substantially orthogonal to the first axis, and a second reflecting surface that deflects and emits the light substantially parallel to the first axis and that faces the first prism, a third prism that deflects the light from the second prism substantially parallel to the incident optical axis, and a positive lens that focuses the light from the second prism, in this order from an object side, the negative lens and the first prism being rotatable about the first axis relative to the second prism.