To provide a plate joining tool used to mutually join steel sheet-concrete composition slabs for use in a structure such as a road bridge, capable of dispensing with an operation from the lower side in construction, and a joining method using the joining tool.

The plate joining tool 1 includes a bolt 2 and a circular stopper 3 which can be elastically suitable for a thread part of the bolt 2 from the side. The stopper 3 includes a projection composed of a part of a thread ridge to be meshed with the thread groove of the bolt 2 on its inner circumference in an integrated manner. In joining of plates by use of this tool, the bolt 2 is inserted to a bolt hole provided in a joint region of a composite plate slab 10 to be joined from the lower surface side, and the stopper 3 is elastically fitted, from the side, to the thread part of the bolt 2 protruded to the upper surface side of the composite plate slab. Such composite plate slabs 10 are carried in a construction site, and mutually butted, a splice plate 15 is placed on the joint region, and the protruding portion of each bolt 2 is inserted to a bolt hole of the splice plate 15 and fastened by a nut 5.