An injection timing control device for a fuel injection pump includes a timer cylinder and a timer piston slidably received in the timer cylinder to define a high-pressure chamber and a low-pressure chamber at opposite axial ends thereof. The high-pressure chamber communicates with a high pressure side, while the low-pressure chamber communicates with a low pressure side and is provided therein with a spring for biasing the timer piston toward the high-pressure chamber. A servo valve having a spool is further provided. Communication between the high-pressure and low-pressure chambers is allowed or prohibited depending on movement of the spool of the servo valve. When the communication is allowed, a fuel pressure in the high-pressure chamber is released into the low-pressure chamber so that the timer piston moves toward the high-pressure chamber due to a biasing force of the spring. This movement of the timer piston is transmitted to a roller ring to retard a fuel injection timing. On the other hand, when the communication is prohibited, the piston moves in the opposite direction so that the fuel injection timing is advanced.