A drilling tool used for excavating a ground to form a borehole, the tool comprising: a casing pipe having a cylindrical shape; an inner bit which is inserted into the casing pipe in a direction of an axial line thereof and of which a distal end portion in the direction of the axial line protrudes from a distal end of the casing pipe; and a ring bit which has an annular shape, is disposed at a distal end portion of the casing pipe so as to be rotatable around the axial line relative to the casing pipe, surrounds the distal end portion of the inner bit, and is capable of engaging with the inner bit around the axial line and from a distal end side of the inner bit in the direction of the axial line, wherein the inner bit is provided with: a supply hole which passes through the inner bit and is open at the distal end portion of the inner bit; and a discharge groove which is formed in an outer peripheral surface of the inner bit and extends in the direction of the axial line, the supply hole is provided with: a distal end blow hole which is open in a distal end surface of the distal end portion of the inner bit; and an outer peripheral blow hole which is open in an outer peripheral surface of the distal end portion of the inner bit, an outer peripheral groove through which the outer peripheral blow hole and the discharge groove communicate with each other is formed in the outer peripheral surface of the inner bit, and the outer peripheral groove is covered with the ring bit from the outside in a radial direction and extends toward the discharge groove from the outer peripheral blow hole so as to become gradually closer to the base end side in the direction of the axial line around the axial line, characterized in that a cross-sectional area of the outer peripheral groove is made to be smaller than a cross-sectional area of the outer peripheral blow hole. a casing pipe having a cylindrical shape; an inner bit which is inserted into the casing pipe in a direction of an axial line thereof and of which a distal end portion in the direction of the axial line protrudes from a distal end of the casing pipe; and a ring bit which has an annular shape, is disposed at a distal end portion of the casing pipe so as to be rotatable around the axial line relative to the casing pipe, surrounds the distal end portion of the inner bit, and is capable of engaging with the inner bit around the axial line and from a distal end side of the inner bit in the direction of the axial line, wherein the inner bit is provided with: a supply hole which passes through the inner bit and is open at the distal end portion of the inner bit; and a discharge groove which is formed in an outer peripheral surface of the inner bit and extends in the direction of the axial line, the supply hole is provided with: a distal end blow hole which is open in a distal end surface of the distal end portion of the inner bit; and an outer peripheral blow hole which is open in an outer peripheral surface of the distal end portion of the inner bit, an outer peripheral groove through which the outer peripheral blow hole and the discharge groove communicate with each other is formed in the outer peripheral surface of the inner bit, and the outer peripheral groove is covered with the ring bit from the outside in a radial direction and extends toward the discharge groove from the outer peripheral blow hole so as to become gradually closer to the base end side in the direction of the axial line around the axial line, characterized in that a cross-sectional area of the outer peripheral groove is made to be smaller than a cross-sectional area of the outer peripheral blow hole. a supply hole which passes through the inner bit and is open at the distal end portion of the inner bit; and a discharge groove which is formed in an outer peripheral surface of the inner bit and extends in the direction of the axial line, the supply hole is provided with: a distal end blow hole which is open in a distal end surface of the distal end portion of the inner bit; and an outer peripheral blow hole which is open in an outer peripheral surface of the distal end portion of the inner bit, an outer peripheral groove through which the outer peripheral blow hole and the discharge groove communicate with each other is formed in the outer peripheral surface of the inner bit, and the outer peripheral groove is covered with the ring bit from the outside in a radial direction and extends toward the discharge groove from the outer peripheral blow hole so as to become gradually closer to the base end side in the direction of the axial line around the axial line, characterized in that a cross-sectional area of the outer peripheral groove is made to be smaller than a cross-sectional area of the outer peripheral blow hole. a distal end blow hole which is open in a distal end surface of the distal end portion of the inner bit; and an outer peripheral blow hole which is open in an outer peripheral surface of the distal end portion of the inner bit, an outer peripheral groove through which the outer peripheral blow hole and the discharge groove communicate with each other is formed in the outer peripheral surface of the inner bit, and the outer peripheral groove is covered with the ring bit from the outside in a radial direction and extends toward the discharge groove from the outer peripheral blow hole so as to become gradually closer to the base end side in the direction of the axial line around the axial line, characterized in that a cross-sectional area of the outer peripheral groove is made to be smaller than a cross-sectional area of the outer peripheral blow hole.