To accurately bore a rivet shaft hole in a plate joined to a plate already bored with a countersunk rivet hole including a countersunk part.

This boring tool has a guide plate material fixing process 300 of fixing two plate materials by being positioned by the countersunk part and a prepared hole by boring the prepared hole in the plate materials bored with a hole including the countersunk hole and joined with a guide plate material being the joining one plate material as a guide, a hole boring template fixing process 600 of fixing a hole boring template provided with a hole for fitting a guide bush for guiding a drill in layers by aligning the drill center with the prepared hole center, and a regular hole boring process 800 of boring the rivet shaft hole by being guide by the guide bush.