A treatment device includes: a treatment portion including an abutment surface which is abutted on the living body tissue, and a back surface opposed to the abutment surface, the treatment portion being capable of applying energy to the living body tissue via the abutment surface, and heat being transferred toward the back surface in accordance with the application of the energy; a cover covering the back surface of the treatment portion; and an adjuster disposed between the treatment portion and the cover, and configured to vary a distance between the treatment portion and the cover, and to adjust an air layer region between the treatment portion and the cover.