A medical device and method are disclosed to be used when indwelling a belt-shaped flexible implant into a living body. The medical device includes: an elongated puncture needle which, by piercing the living body, forms a primary insertion hole in the living body and a sheath into and out of which the puncture needle can be inserted and removed and which, by passing through the primary insertion hole in an assembled state with the puncture needle inserted therein, makes the primary insertion hole into a secondary insertion hole so as to permit the implant to be inserted in and passed through the secondary insertion hole. The puncture needle has a puncture needle-side expanding section which, when forming the primary insertion hole, expands the primary insertion hole to the same degree as the width of the implant.
