To provide a tool which is for use in boring an expanded-diameter hole around an existing hole formed in a wall surface, a top board, or the like of a building made of mortar, wood, or the like, and facilitates the boring of the expanded-diameter hole irrespective of the diameter and shape of the circumference of the existing hole.

An expanded-diameter boring device by a core drill has a conical cup detachably attached to a core body thereof, and the protrusion degree of the conical cup is adjusted at a location of a cutting blade of the core body so as to match with an existing hole diameter. Therefore when expanded-diameter hole boring work is carried out, by matching the conical cup with the existing hole and fitting the former into the latter, the entire body of the core drill does not fall in an unstable state, and the expanded-diameter hole boring work is facilitated.