PROBLEM TO BE SOLVED: To provide a medical device for puncturing a desired living tissue only even when a living tissue to be punctured is near a living tissue not to be punctured.SOLUTION: The medical device has: a long catheter 30； a puncture part 2 which is provided at a tip of the catheter 30 and punctures a living tissue； an extended member 66 which is provided at the tip of the catheter 30 and is extensible； and a guide groove 55 which is provided with a specific length from the tip of the catheter 30 toward the end and secures a space where the extended member 66 can be deformed at the tip of the catheter 30 while guiding the extended member 66.
