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