A damping force variable valve includes an improved spool rod to minimize internal twisting, a simplified internal channel to obtain a reliable damping force at low speed, and has a simplified overall structure and a reduced number of components to enhance productivity. In one embodiment, a shock absorber including a damping force variable valve is provided. A damping force variable valve includes a retainer having a main body connected at a central portion thereof to a high pressure part or portion of a shock absorber and having an outer peripheral region enlarging outwardly, and a spool rodintegrally extending from the central portion and formed at a center thereof with a hollow portion having a spool inserted therein. The variable valve further includes a main disc disposed adjacent the retainer, a solenoid unit provided adjacent the spool rod and including a push rod for moving the spool when electrical power is applied, and an operating block installed adjacent the solenoid unitand including an enlarged portion coupled to the outer peripheral region of the main body.