To provide a tool which alleviates the manpower of excavating operation, and simultaneously can shorten operating time.

The tool has a rotation driving part 2, a rotary excavating cutter 3 having a disk-shaped rotational form which is provided at the end of a driving shaft 11 extending from the driving part, and a handgrip 4 provided in such a manner that in the state of downwardly positioning the excavating cutter joined to the rotation driving part, the disk-shaped underside can be moved and operated in the horizontal direction along a horizontal plane. A cover 5 which is formed integrally with the handgrip and covers the upper side and the most part of the periphery of the excavating cutter is provided so as to expose the excavating cutter in accordance with the groove depth of excavating cutting. The cover is provided with a stopper 6 cable of controlling the groove depth which regulates the groove depth in contact with the jamb surface. The lower end 22 of the cover is provided so as to protrude at a height equal to or slightly below the height of the underside of the disk-shaped rotational form of the excavating cutter.