To provide a tool for attaching a license plate and its attaching structure, preventing the license plate from being stolen, easily and at low costs, and enabling easy attachment of the license plate to a vehicle body.

When a cap is fitted to a shaft part 23 of an attaching pin 22, since a retaining member 10 exists between a peripheral wall of a shaft part insertion hole 16 and the shaft part 23, the fitted cap 15 is not pulled out from the shaft part 23. Since a tip part 23b of the shaft part 23 is covered with a back wall of the cap 15, the shaft part 23 protruded from a back of a bumper 75 is not pulled out with pliers. This structure prevents the license plate 70 from being stolen, easily and at low costs, and makes it easy to attach the license plate 70.