A medical device and a treatment method, in which energy is accurately applied to a target site by suppressing a positional displacement between an expansion body and a biological tissue. The medical device includes an elongated shaft portion and an expansion body disposed in a distal portion of the shaft portion and configured to expand and contract in a radial direction. The expansion body includes a holding portion having a proximal side holding portion and a distal side holding portion configured to hold the biological tissue, and a movable portion configured to open and close the holding portion.