Provided is a cutting instrument including: an elongated member including a tubular portion 105 and a shaft-like portion 101 inserted m 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 and a projected position 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 an 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.
