To provide a reinforcing bar joining device provided with a constitution capable of keeping an alignment precision when the reinforcing bars are joined regardless of the occurrence of the assembly error of parts with the lapse of time.

When a sliding clamping part 13 which is integrated through a connecting part 12A which is possessed in a sliding body 12 which is slid in a hollow body is rocked by taking the sliding body 12 as a supporting point in order to the alignment of the reinforcing bars, a guide part 15 having a surface of curvature having the center on the outside in the thickness direction and a surface with which surface contact is possible and which is in parallel to and flush with the surface of the connecting part on the opposite side of the surface of curvature is provided on a connecting part 12A and rotation in the opposite positions of eccentric adjusting screws 16, 16' which are used when rocking a sliding clamp part 13 is made so that the center of rotation in that position is not present and also the rotation of the connecting part in the position is stopped by the flush contact surface.