A connector plate makes it possible to join together carriages with high rigidity and much load rating no matter lengthwise dimensions of the carriages. The carriage has at leading and trailing ends thereof the connector plates identical in structure. The connector plate on a front surface thereof has a tenon or projection and a mortise which are spaced widthwise from each others. The adjoining carriages are joined together after the connector plates have been placed each other in a relation the their front surfaces have made direct engagement face-to-face with each other with the tenon having fit into the mortise to make the tenon-and-mortise joint.