PROBLEM TO BE SOLVED: To cut an operation part by an impact force of liquid without lowering selection cut performance.SOLUTION: Liquid is injected at a flow rate higher than a preceding flow in continuation to the preceding flow which is injected at a constant flow rate. Since the preceding flow flows at a constant flow rate, when the preceding flow abuts on an obstacle such as a blood vessel, the preceding flow flows so as to wrap around an opposite side of the obstacle along a surface of the obstacle by a Coanda effect. Here, by injecting a main body flow in continuation to a continuous flow, the main body flow can be made to wrap around the opposite side of the obstacle along a flow of the preceding flow, and thereby the operation part can be selectively cut while avoiding the obstacle. Also, by injecting the main body flow at a flow rate higher than the preceding flow, the main body flow abuts on the operation part at the flow rate higher than the preceding flow, and thereby even if the preceding flow abuts on the operation part before the main body flow does, the main body flow can apply an impact force to the operation part. Consequently, the operation part can be cut by the impact force of liquid without lowering selection cut performance.
