A cage interposed between a pair of vertebral bodies, wherein both left and right side surfaces of a cage body are formed in a smooth surface, both upper and lower surfaces of the cage body are provided with a large number of projections arranged in a longitudinal direction and a lateral direction, tip end surfaces of the large number of projections substantially match with a virtual arc curved surface formed around an axis in the longitudinal direction, and angles of inner end edges of the projections close to left and right side surfaces are more acute. A longitudinal through hole is formed in the cage body in its longitudinal direction so as to penetrate the cage body, and both left and right side surfaces and both upper and lower surfaces of the cage body are provided with communication holes which are in communication with the longitudinal through hole. The communication holes are circular holes having a common diameter.