A medical instrument includes a medical needle configured to puncture a tissue, and a tissue-supporting member configured to be inserted into the lumen of the medical needle and supports the tissue. The medical needle includes an insertion portion that has a longitudinal axis, a lumen that is formed along the direction of the longitudinal axis of the insertion portion, an opening portion that is formed at the distal end of the insertion portion while being connected to the lumen, and a puncturing portion that is formed in the periphery of the opening portion and has a pair of projections sharply protruding in a direction extending toward the distal end from the proximal end of the insertion portion.