The upper and lower surfaces 5 and 7 of an intervertebral spacer 1 inserted between the vertebrae of a spine are formed in the shape of a curved surfaces 21A and 21B each having a top portion at the middle portion in the back and forth direction. The above-mentioned upper and lower surfaces 5 and 7 are provided with a plurality of claw portions 17 for preventing withdrawal. Further, the intervertebral spacer 1 is formed in such a way that a tip portion in the direction of insertion is tapered.