A medicine injection device (10) is provided with a medicine ejection unit (14) having a flow-in port (31) of liquid (19) on the proximal side of a tube body (24), and a pressure generation unit (16) which includes a pressure generating chamber (18) for generating pressure acting upon a working chamber (30) of the medicine ejection unit (14) through the liquid (19) and which ejects medicine from an ejection port (20) by making a floating gasket (26) of the medicine ejection unit (14) progress depending on aforesaid pressure. The pressure generation unit (16) includes a pressure portion cylinder (34) provided inside the pressure generating chamber (18), a first pusher (38) provided with a hollow gasket (36) for defining the inner volume of the pressure generating chamber (18) by sliding inside the pressure portion cylinder (34) in a liquid-tight state and a second pusher (40) which generates the pressure inside the pressure generating chamber (18) by pressing a distal end head portion (40c) into the inside of the pressure generating chamber (18), and the pressure portion cylinder (34) and the first pusher (38) are coupled through an engagement mechanism (42) in which fixation and fixation release are possible for each other for every predetermined pitch in the axial line direction thereof.
