A core drilling machine having an axial extending hollow drill shaft supported rotatably in a housing and rotated by means of a drive unit; a coaxial tool holder receiving an axially displaceable drill bit, the tool holder having a locking mechanism securing the drill bit in a rotationally fixed manner to the hollow drill shaft; wherein the hollow drill shaft is suitable for receiving a shaft of an interchangeable centre drill, the center drill axially supported against a first spring element; and an axial stop element preventing in a first position a complete displacement of the center drill into the interior of the hollow drill shaft and allows in a second position a complete displacement of the center drill into the interior of the hollow drill shaft; and an activation device for activating the first or second positions of the stop element.