The invention provides a fixing structure having an inner wall forming a through hole around a central axis and a first rib protruded from the inner wall toward the central axis along a radial direction of the through hole with an extending direction parallel to an axial direction of the central axis, and a projection apparatus including an illumination system, a light valve, and a projection lens. The illumination system includes the fixing structure, an optical device, and a light source illuminating the optical device to form an illumination beam. The optical device is placed in the through hole and interfered by the first rib to be fixed in the through hole. The light valve is located on a transmission path of the illumination beam and converts it into an image beam. The projection lens is located on a transmission path of the image beam and projects the image beam.