A device for forming an expanded hole section in the inner wall of a joining hole, which is formed to a necessary depth in the joining surface of a wooden member, the device being provided with: a cutter body having a cutting tool, which is formed with a diameter that fits in the joining hole and the blades of which are configured to move between a stowed position that fits within the diameter and an extended position in which the blades project out from the diameter, a reciprocating member, which is moved in the axial direction by an external force and which rotates as a unit in the circumferential direction of the axis, and a linking mechanism for changing the reciprocating movement of the reciprocating member into the movement of the cutting tool; a tip bearing, which is provided on the tip end of the cutter body, fits in the deepest part of the joining hole, and serves as a tip end bearing; and a rotational force-transmitting element for transmitting rotational force in the circumferential direction of the axis to the reciprocating member. In the device, the present invention makes it possible to perform movements during rotation of the cutting blades more smoothly and stabilizes movement in the direction of rotation of the cutting blades during cutting.