A medical device and a treatment method are disclosed which allow easy formation of a hole using energy and easy expansion of the formed hole without loss of tissue and allow an increase in working efficiency. A medical device is disclosed for forming a hole in a tissue in a living body and expanding the hole, the medical device including: a dilator having a first lumen formed inside and having a tip portion in which the first lumen opens on a distal side； and an output unit that outputs energy for denaturing the tissue and forming a hole, in which the output unit is disposed on a tip portion and formed discontinuously in the circumferential direction of the tip portion, and the outer diameter of the tip portion gradually decreases toward the distal side.
