A medicine injection device includes a medicine ejection unit having a fluid flow-in port for receiving liquid, and a pressure generation unit which includes a pressure generating chamber to generate pressure acting upon a working chamber of the medicine ejection unit through the liquid and which ejects medicine by moving a gasket based on the applied pressure. The pressure generation unit includes a pressure portion cylinder in the pressure generating chamber, a first pusher provided with a hollow gasket defining the inner volume of the pressure generating chamber and a second pusher which generates the pressure inside the pressure generating chamber by pressing a distal end head portion into the inside of the pressure generating chamber. The pressure portion cylinder and the first pusher are coupled through an engagement mechanism in which fixation and fixation release are possible at different pitches in the axial direction.
