An injection device including a hollow barrel, a needle unit and a push rod is provided. The hollow barrel has a connecting portion. The needle unit is disposed at the connecting portion and has an engaging structure, and the engaging structure is located in the hollow barrel. The push rod is movbly disposed in the hollow barrel and has a pushing portion, and the pushing portion faces the connecting portion and has a plurality of elastic hooks. The push rod is adapted to move toward the connecting portion to push a liquid in the hollow barrel to be injected through a needle of the needle unit, and the push rod is adapted to keep moving toward the connecting portion, such that the elastic hooks engage with the engaging structure.一種注射裝置，包括一中空筒體、一針頭組及一推桿。中空筒體具有一銜接部。針頭組配置於銜接部且具有一卡合結構，卡合結構位於中空筒體內。推桿可移動地配置於中空筒體內且具有一推動部，推動部朝向銜接部且具有多個彈性卡勾。推桿適於往銜接部移動以推動中空筒體內的液體通過針頭組的針體而注射出，且推桿適於往銜接部繼續移動以使彈性卡勾卡合於卡合結構。
