To solve the problem wherein in the conventional expansion operation for a ceiling hole on the ceiling having a sprinkler head to be mounted, the hole to be expanded is not concentric circle with the sprinkler head due to the tools such as a saw, the operation requires a lot of hard work during operation, and deteriorates operating efficiency.

A hole saw 10 expands the ceiling hole provided on the ceiling 50 drillingly provided with the sprinkler head 20. The hole saw 10 is equipped with a cylindrical rotating shaft body 1 and a cutting part 2 which is positioned at the outside of the rotating shaft body 1 and coaxial to the rotating shaft body 1. The cutting part 2 rotates around the sprinkler head 20 by clearance fitting the sprinkler head 20 to the inside of the rotating shaft body 1, and the ceiling hole 50b after expansion becomes a circle with the sprinkler head 20 as an almost the center.