Provided is a damping force variable shock absorber in which an additional valve is installed in a rod guide so as to implement performance that could not be implemented by a main valve. The damping force variable shock absorber includes: a base shell in which a damping force variable valve assembly is attached to an outside thereof; an inner tube which is installed inside the base shell and in which a piston rod is installed to be movable in a length direction; a piston valve which is connected to one end of the piston rod to partition an internal space of the inner tube into a rebound chamber and a compression chamber; a separator tube which is installed to partition a space between the base shell and the inner tube into a low-pressure chamber and a high-pressure chamber.