A cutting instrument comprising: an elongated member including a tubular portion, and a shaft-like portion inserted in the tubular portion; a blade provided at an end of the elongated member, the blade including a pivot axis in a center thereof, and including an edge (201 b, 201 c) at each of two positions with the pivot shaft interposed therebetween; an operation portion provided at another end of the elongated member, the operation portion linearly moving the tubular portion and the shaft-like portion relative to each other; and a motion mechanism that converts a relative linear movement of the tubular portion and the shaft-like portion resulting from an operation of the operation portion into pivoting of the blade, the blade thereby pivoting between a housed position where the blade is housed in the elongated member, and a projected position where the edges are projected in two directions outside the elongated member, wherein the tubular portion of the elongated member is moved away from the operation portion while the elongated member being rotated with the edges of the blade projected from the elongated member, thereby drilling a hole having a diameter larger than an outer diameter of the tubular portion wherein the motion mechanism includes an outer peripheral surface of the blade, and an inner surface in an axial direction of a slit (105a, 105b) provided in the tubular portion, the slit being formed so as to have a width larger than a thickness of the blade; and wherein the outer peripheral surface abuts the inner surface in the axial direction of the slit, the blade thereby pivoting between the housed position and the projected position. an elongated member including a tubular portion, and a shaft-like portion inserted in the tubular portion; a blade provided at an end of the elongated member, the blade including a pivot axis in a center thereof, and including an edge (201 b, 201 c) at each of two positions with the pivot shaft interposed therebetween; an operation portion provided at another end of the elongated member, the operation portion linearly moving the tubular portion and the shaft-like portion relative to each other; and a motion mechanism that converts a relative linear movement of the tubular portion and the shaft-like portion resulting from an operation of the operation portion into pivoting of the blade, the blade thereby pivoting between a housed position where the blade is housed in the elongated member, and a projected position where the edges are projected in two directions outside the elongated member, wherein the tubular portion of the elongated member is moved away from the operation portion while the elongated member being rotated with the edges of the blade projected from the elongated member, thereby drilling a hole having a diameter larger than an outer diameter of the tubular portion wherein the motion mechanism includes an outer peripheral surface of the blade, and an inner surface in an axial direction of a slit (105a, 105b) provided in the tubular portion, the slit being formed so as to have a width larger than a thickness of the blade; and wherein the outer peripheral surface abuts the inner surface in the axial direction of the slit, the blade thereby pivoting between the housed position and the projected position.