Abstract: Disclosed is an injection device for properly dissolving pharmaceutical agents contained in a two-compartment syringe having slidable front and rear walls, and then performing portionwise multiple injections. The injection device comprises a front casing designed to enclose and hold the two-compartment syringe with a double-ended needle attached to the front end of it, a rear casing designed to receive the thus held two-compartment syringe being inserted in the rear-end first manner, and to lock and hold the front casing, a piston rod placed in the rear casing and designed to push from behind and carry forward the slidable rear wall, wherein the piston rod can be pushed forwardly from behind the rear casing, and a spring provided so that, when the front end of the piston rod is moved backward, the spring may be once compressed and then push the piston rod to a forward position again, and forwardly drives the slidable rear wall to abut on the slidable front wall.