There is provided an endoscope illuminating optical system and an endoscope optical system unit which enable to change a direction of irradiation of illumination light to two directions, without bending an endoscope even in a narrow space.
In an endoscope illuminating optical system 10 includes a light-emitting section 11 and a first optical-path deflecting prism group PG1 which takes in illumination light emitted from the light-emitting section 11, and irradiates light to an subject upon deflecting an optical path, the first optical-path deflecting prism group PG1 includes three prisms in order from a subject side which are, a first prism P1, a second prism P2, and a third prism P3, and the first prism P1, the second prism P2, and the third prism P3 are disposed to be mutually adjacent, and a direction of irradiation A of the illumination light is let to be variable to a first direction DR1 by rotationally moving the first prism P1 with respect to the second prism P2, and the direction of irradiation A of the illumination light is let to be variable to a second direction DR2 which differs from the first direction DR1, by rotationally moving the first prism P1 and the second prism P2 integrally with respect to the third prism P3.