To prevent idle hammering by feeding part of water, fed to a bit, to a void formed at a fit-in part of a head stem and a head sleeve.

This boring tool is provided with a head stem supplied with impact and rotation by a machine drill, a head sleeve 3 fitted to the peripheral part of the head stem 2 slidably in an axial direction and with no possibility of mutual rotation, a preceding bit 5 mounted on the tip part of the head sleeve 3, and a wing bit 6 accommodated in a radially expanding/contracting state in a storage part formed at the side face part of the head sleeve 3. In this case, the head stem 2 is provided with a water hole 26 for feeding high pressure water to the preceding bit 5, and a through hole 28 is provided to connect the water hole 26 to a void 8 formed at a fit-in part of the head stem 2 and head sleeve 3. The head sleeve 3 is pressed forward by water pressure so as to suppress reaction of the head sleeve 3 at the time of hammering.