An auto-injector for delivering a liquid medicament comprising: a housing arranged to contain a syringe with a piston for sealing the syringe and displacing the medicament, the housing having a distal end and a proximal end intended to be applied against an injection site； a plunger unit arranged between the piston and the distal end of the housing, the plunger unit comprising a threaded internal surface for receiving and engaging a threaded rod element； and a tensioned elastic element fixed to the threaded rod element, the elastic element and the threaded rod element being fixed against rotation in an initial state, wherein when the elastic element is rotationally released, the threaded rod element rotates in a first direction, causing the plunger unit to translate towards the proximal end of the housing and thereby pushing the piston towards the proximal end of the housing to displace the medicament.一種用於輸送液體藥劑的自動注射器，包括：殼體，其布置成包含注射筒，該注射筒包括用於密封注射筒並且排出藥劑的活塞，該殼體具有遠端和近端，近端擬用來靠在注射部位上；柱塞單元，其布置在活塞和殼體的遠端之間，並且包括用於接收和接合螺紋桿元件的螺紋內表面；張緊彈性元件，該彈性元件固定到螺紋桿元件，在初始狀態下，該彈性元件和螺紋桿元件被固定以防旋轉，其中當彈性元件在旋轉方向上被釋放時，螺紋桿元件在第一方向上旋轉，導致柱塞單元朝向殼體的近端平移且由此將活塞推向殼體的近端，排出藥劑。
