An injection needle assembly 2 is configured to be tapered-fitted to a drug discharge portion 6. The injection needle assembly 2 includes a holder 13 and an elastic member 16. The holder 13 holds a needle tube 12 in a state where a rear end 12b of the needle tube 12 is projected. The elastic member 16 is arranged within a fitting portion 18 and includes a closely-contacting portion 47 that adhesively holds the needle tube 12 in a state where the needle tube 12 is projected to the side of a fitting hole, and a deformable portion 40 that extends from the closely-contacting portion 47 to the side of the fitting hole and deforms when the drug discharge portion 6 abuts thereon. Further, the length of the deformable portion 40 falls within a range between 35% and 50% of the length of the elastic member 16, and the length of a portion of the needle tube 12 projecting from the closely-contacting portion 47 to the side of the fitting portion 18 falls within a range between 45% and 245% of the length of the deformable portion 40.本發明的目的在於提供抑制了液體流通不良的注射針組裝體、以及具備該注射針組裝體的藥劑注射裝置。在與藥劑排出部6錐形嵌合的注射針組裝體2中，該注射針組裝體2包括：保持部13，以使針管12的後端12b突出的狀態來保持針管12；以及彈性構件16，具備密接部47及變形部40，且設置於嵌入部18內，所述密接部47密接保持向嵌入口側突出的狀態的針管12，所述變形部40從密接部47向嵌入口側延伸，在藥劑排出部6抵接時發生變形。而且，變形部40的長度相對於彈性構件16的長度為35%以上且50%以下的範圍，從密接部47向嵌入部18側突出的針管12的長度相對於變形部40為45%以上且245%以下的範圍。
