An electric power steering system includes a second shaft coupled to a first shaft via a torsion bar, a basic driver torque estimation unit (extended state observer) configured to estimate a basic driver torque by using a disturbance observer based on a torsion bar torque and a rotation angle of the second shaft, a gravity torque calculation unit configured to calculate, by using a rotation angle of a steering wheel, a gravity torque applied to the first shaft by gravity acting on the center of gravity of the steering wheel, and a driver torque estimation unit configured to estimate a driver torque by using the basic driver torque estimated by the basic driver torque estimation unit and the gravity torque calculated by the gravity torque calculation unit.