Disclosed is a method of assembling a medicament injection device (1) including the following steps: a step of integrally fixing a needle tube (3) to a needle hub (4) by high-frequency welding a step of radiation sterilizing the needle tube (3) and the needle hub (4) a step of subjecting a container (2) to be filled with a medicament (M) to a sterilization treatment which involves generation of heat a step of aseptically transporting the needle hub (4) which has been subjected to the radiation sterilization into an isolator device (H) a step of fixing the needle hub (4) to the container (2) in a sterile environment in the isolator device (H) and a step of filling the container (2) fixing the needle hub (4) with the medicament (M) in the sterile environment.
