Abstract: Provided is a damping force variable valve of a shock absorber. The damping force variable valve of the shock absorber includes: a retainer formed with an inflow passage into which a working fluid is introduced from the shock absorber; a valve body disposed behind the retainer; an elastic member pressurizing the valve body in a direction of stopping the inflow passage, behind the valve body; a spool guide formed with a back pressure control passage inducing some of the working fluid in front of the valve body into a back pressure chamber behind the valve body; and a spool moving forward and backward according a change in current of a solenoid to control an opening of the back pressure control passage, wherein the valve body moves backward from a position where the inflow passage stops to form a discharge passage between the valve body and the retainer, and the size of the discharge passage varies by the pressure of the working fluid to control the discharge extent of the working fluid.