The invention provides a medical instrument and an injection needle assembly reliably attached to a drug discharge portion without any breakage of a locking mechanism and also provides a drug injection apparatus including the injection needle assembly. In the injection needle assembly of the invention, a drug discharge portion of a syringe is fitted into a fitting portion by taper-fitting and is fixed to the syringe by a locking mechanism. Further, a restriction portion is provided which restricts an operation in which the drug discharge portion is pushed into the fitting portion while the restriction portion contacts the locking mechanism when the drug discharge portion is inserted into the fitting portion and a first screw portion of the locking mechanism is threaded into a second screw portion of the fitting portion. By the restriction portion, an operation in which the drug discharge portion is pushed into the fitting portion is restricted.
