A drug injection syringe comprising: an injection needle assembly including an injection needle, a needle hub for supporting the injection needle, an elastic body mounted on the needle hub, the injection needle being inserted in the elastic body, and a fitting sleeve accommodating the needle hub and the elastic body and including an opened fitting hole formed in a female taper shape becoming thinner toward a needle tip; and a syringe body including an outer tube for containing drug sealed by a gasket inserted in the outer tube and a drug discharge tube provided on a front end side of the outer tube to be formed in a male taper shape to fit in the female taper, the syringe body being configured to be attached to the injection needle assembly by pushing the drug discharge tube into the fitting hole by press fitting, wherein the elastic body has a pocket opening to a contact surface that makes contact with a distal end of the drug discharge tube, and couples together the drug discharge tube and the injection needle in a liquid-tight manner by compressively deforming, by the press fitting, at a protruding portion provided on a periphery of the pocket to stretch against an inner wall of the fitting hole to provide liquid-tightness, wherein a gap is provided between the elastic body and the inner wall of the fitting hole, and allows the elastic body to compressively deform to stretch by the press fitting to provide liquid-tightness, characterized in that the protruding portion of the elastic body is configured to stretch inward into an upper pocket of the elastic body for decreasing the volume of the upper pocket by compressive deformation. an injection needle assembly including an injection needle, a needle hub for supporting the injection needle, an elastic body mounted on the needle hub, the injection needle being inserted in the elastic body, and a fitting sleeve accommodating the needle hub and the elastic body and including an opened fitting hole formed in a female taper shape becoming thinner toward a needle tip; and a syringe body including an outer tube for containing drug sealed by a gasket inserted in the outer tube and a drug discharge tube provided on a front end side of the outer tube to be formed in a male taper shape to fit in the female taper, the syringe body being configured to be attached to the injection needle assembly by pushing the drug discharge tube into the fitting hole by press fitting, wherein the elastic body has a pocket opening to a contact surface that makes contact with a distal end of the drug discharge tube, and couples together the drug discharge tube and the injection needle in a liquid-tight manner by compressively deforming, by the press fitting, at a protruding portion provided on a periphery of the pocket to stretch against an inner wall of the fitting hole to provide liquid-tightness, wherein a gap is provided between the elastic body and the inner wall of the fitting hole, and allows the elastic body to compressively deform to stretch by the press fitting to provide liquid-tightness, characterized in that the protruding portion of the elastic body is configured to stretch inward into an upper pocket of the elastic body for decreasing the volume of the upper pocket by compressive deformation.