To eliminate a tendency of a drill to corrosion and breakage by projecting one of cutting blades of an insert mutually overlapping partially in the radial direction in cutting work to be soldered to agroove over the head side surface on the radial direction side, and terminating the other cutting blades inside in the radial direction of the head side surface.

A cutting blade 64 of an insert 60 soldered to a groove formed in the front face 52 is turned to the tangential direction T side opposite to the tangential directions T' of cutting blades 62, 66, the outer and inner cutting blades 62, 64 are expanded backward in the longitudinal direction and outward in the radial direction, and the center cutting blade 66 is expanded backward in the longitudinal direction and inward in the radial direction and overlaps the shaft partially in the radial direction. In cutting work, the inner cutting blade 64 partially overlaps an area between the outer and central cutting blades 62, 66 in the radial direction, the outer and central cutting blades 62, 66 are put in the rear of the periphery of an large opening part 48 with respect to the rotating direction R in cutting, and the inner cutting blade 64 is put in the rear of the periphery of a small opening part 50.