1. An injection needle unit comprising: a needle tube having a needle tip for piercing the skin； a needle sleeve including a first element that holds the needle tube and a second element into which the outlet portion of the syringe is inserted； and an elastic element located inside the second element to create liquid tight tight contact with the inner surface of the second element, while the elastic element has an insertion hole into which the needle tube is inserted. 2. The injection needle block according to claim 1, in which the elastic element has a thrust surface that abuts against the distal end of the outlet of the syringe. The injection needle unit according to claim 2, in which the elastic element has an spaced portion from the side of the abutment surface, which forms the opening of the insertion hole on the abutment surface and which is located at a distance from the outer peripheral surface of the needle tube. The injection needle unit according to claim 2, in which the elastic element has an end surface that is located on the side opposite to the supporting surface, and an spaced portion from the side of the end surface, which forms the opening of the insertion hole on the end surface, and which is located at a distance from the outer the peripheral surface of the needle tube. 5. The injection needle unit according to claim 1, wherein the elastic member has a flange portion that is sandwiched between the first member and the second member. The injection needle unit according to claim 1, in which the elastic element is provided with a part of the valve element on the side of the needle, which is pressed against the outer peripheral surface of the needle tube under pressure exerted on it by a liquid1. Блок инъекционной иглы, содержащий:трубку иглы, имеющую острие иглы для прокалывания кожи；втулку иглы, включающую в себя первый элемент, который удерживает трубку иглы, и второй элемент, в который вставляется выпускная часть шприца； иупругий элемент, располо
