An injection needle assembly is an assembly used by being attached to an injector including an injection barrel filled with medicine and a needle tube, and is provided with a sticking needle tube having a needle tip to be stuck into a skin, a needle holding portion, an engagement portion and a connection portion. The needle holding portion holds the sticking needle tube and a fit tube of the engagement portion is engaged with a needle hub of the injection barrel. The connection portion includes a luminal portion in which there are arranged a proximal end on the opposite side of the needle tip of the sticking needle tube and a needle tip (ejection port) at the needle tube of the injector, and the sticking needle tube and the needle tube communicate with each other in a liquid tight manner.