Abstract: An injection device is disclosed which is designed to receive therein a two-compartment syringe, and to be used to mix the pharmaceutical agents and then conduct injections. The injection device comprises a cylindrical barrel portion so configured that the two-compartment syringe can be inserted therein in a rear-end first manner, a piston rod for pushing the slidable rear wall to advance, a front spring that is compressed, when the two-compartment syringe is inserted, and then push back the piston rod in the forward direction, and a sleeve which can be shifted back and forth and is provided by screw engagement with the outer surface of the rear part of the barrel portion to block the forward movement of the piston rod.