[Problem to be Solved] Achieving cutting with a high cutting efficiency and a high stability. [Solution] Provided is a cutting instrument including: an elongated member including a tubular portion 105 and a shaft-like portion 101 inserted in the tubular portion 105 a blade 201 including two edges, the blade 201 being provided at an end of the elongated member and being pivotable between a housed position where the blade 201 is housed in the elongated member and a projected position where the two edges are projected outside the elongated member an operation portion 102 to 104 providing at another end of the elongated member, the operation portion 102 to 104 moving the tubular portion 105 and the shaft-like portion 101 relative to each other and a motion mechanism that converts a relative movement of the tubular portion 105 and the shaft-like portion 101 resulting from an operation of the operation portion 102 to 104 into pivoting of the blade between the housed position and the projected position, wherein the cutting instrument moves toward the operation portion while rotating in its entirety with the two edges of the blade 201 projected on opposite sides of the tubular portion 105, thereby drilling a hole with a diameter larger than an outer diameter of the tubular portion 105.
