The present invention provides a needle assembly which prevents a liquid passing failure, and a drug injection device which includes this needle assembly. A needle assembly according to the present invention includes: a retaining portion which holds a needle tube in a state wheproximalear end of the needle tube protrudes； and an elastic member which includes a close contact portion which closely contacts and retains the needle tube which protrudes toward a fitting opening, and a deformation portion which extends from the close contact portion toward the fitting opening and deforms when the drug ejection portion comes into contact with the elastic member, and which is provided in a fitting portion. Further, the length of the deformation portion is in a range of 35% or more and 50% or less with respect to the length of the elastic member, and the length of the needle tube which protrudes from the close contact portion toward the fitting portion is in a range of 45% or more and 245% or less with respect to the deformation portion.
