Injection needle unit, storage case, injection needle assembly and drug injection device

An injection needle unit (4) comprises a needle pipe (5), a needle hub (6), an engaging portion (25) and a storage case (8). The needle hub (6) holds the needle pipe (5) and has a thread groove (35) which screws with a screw portion (53) of a syringe (3). The engaging portion (25) is provided on a side face part of the needle hub (6). The storage case (8) comprises a tubular case main body (61) which accommodates the needle hub (6) and an engaged portion (67) which is provided inside the case main body (61) and engages with the engaging portion (25). The engaging portion (25) and engaged portion (67) are removed from engagement with each other when a predetermined load is applied to the axial direction.