An injector is provided. The injector includes a case body, a dose preset element, a first engagement element, a second engagement element, an elasticity element, and a button. The dose preset element is used to preset a dose. The first engagement element is connected with the dose preset element. The elasticity element is connected between the first engagement element and the second engagement element. When the elasticity element is in an extension state, the first engagement element and the second engagement element are separated. When the button is pushed, the elasticity element is entered into a compress state to combine the first engagement element and the second engagement element, and then push the dose preset element.
