To provide high cutting performance by reducing banishing torque for a boring tool having a guiding pad.

An auxiliary cutter 7 is formed in at least one spot in the outer peripheral direction rear side of a first cutter 4 provided in a cutter 1. At this time, an opening angle θ formed by the first cutter 4 and the auxiliary cutter 7 around the axial center of a boring tool is set to 100° to 170°. Thus, since a guiding pad portion for supporting a combined force between a centrifugal force and a cutting force is changed to a cutter, since fine cutting is to be performed banishing torque is reduced.