A cutting instrument including: an elongated member including a tubular portion and a shaft-like portion inserted in the tubular portion a blade including two edges, the blade being provided at an end of the elongated member an operation portion moving the tubular portion and the shaft-like portion relative to each other and a motion mechanism that converts a relative movement of the tubular portion and the shaft-like portion resulting from an operation of the operation portion into pivoting of the blade between a housed position and a projected position.
