Disclosed is a method of assembling a medicament injection device including the following steps: a step of integrally fixing a needle tube to a needle hub by high-frequency welding a step of radiation sterilizing the needle tube and the needle hub a step of subjecting a container to be filled with a medicament to a sterilization treatment which involves generation of heat a step of aseptically transporting the needle hub which has been subjected to the radiation sterilization into an isolator device a step of fixing the needle hub to the container in a sterile environment in the isolator device and a step of filling the container fixing the needle hub with the medicament in the sterile environment.
