To provide a tool capable of easily enlarging the diameter of an existing hole irrespective of the diameter and the shape of the hole in a drilling operation in which existing holes formed in the wall surface and the ceiling board of mortar and wood buildings are enlarged in diameter.

The hole diameter enlarging device such as a core drill includes a loading part 1 (1A, 1B) so extensible as to be arranged while being fitted into the existing hole. A holding hole is so formed near the center of the loading part 1 that a center drill of a core drill or a center nail can be holdingly inserted thereinto to open a hole larger in diameter than the existing hole. Consequently, the entire part of the hole diameter enlarging device such as a core drill is not made instable, and the diameter enlarging operation can be easily performed.