An object of the present invention is to provide an injection needle assembly in which poor fluid passage is suppressed and a drug injection device including the injection needle assembly. The injection needle assembly (2) of the present invention includes a holding portion (13) that holds the needle tube (12) in a state in which the rear end (12b) of the needle tube (12) protrudes, and a state in which the needle tube (12) protrudes toward the fitting inlet. A close contact portion (47) for tightly holding the needle tube (12), and a deformable portion (40) extending from the close contact portion (47) toward the fitting inlet and deformed when the medicine discharge portion (6) comes into contact therewith. And an elastic member (16) provided in the fitting portion (18). The length of the deformable portion (40) is in the range of 35% to 50% with respect to the length of the elastic member (16), and the needle tube protrudes from the contact portion (47) toward the fitting portion (18). The length of (12) is in the range of 45% to 245% with respect to the deformed portion (40).通液不良が抑制された注射針組立体、及び、その注射針組立体を備える薬剤注射装置の提供を目的とする。 本発明の注射針組立体（２）は、針管（１２）の後端（１２ｂ）を突出させた状態で針管（１２）を保持する保持部（１３）と、嵌入口側に突出した状態の針管（１２）を密着保持する密着部（４７）、及び、密着部（４７）から嵌入口側に延出し、薬剤排出部（６）が当接した際に変形する変形部（４０）を備え、嵌入部（１８）内に設けられた弾性部材（１６）とを有する。そして、変形部（４０）の長さが、弾性部材（１６）の長さに対して３５％以上５０％以下の範囲であり、密着部（４７）から嵌入部（１８）側に突出した針管（１２）の長さが、変形部（４０）に対して４５％以上２４５％以下の範囲にある。
