An electric power steering system, comprising: a power column including an electric motor and a speed reduction mechanism that amplifies motor torque of the electric motor; an assist torque command value calculation unit that sets an assist torque command value of the electric motor; a plant observer that calculates an estimated value ( θ̃) of a rotation angle of the power column and an estimated value ( μ̃N ( d̃h)) of a frictional force that is generated in the power column, by using an observer model and a frictional force estimating unit which outputs said estimated value ( μ̃N ( d̃h)) of a frictional force, wherein in the observer model it is assumed that no friction is generated in the power column and whose characteristic is corrected by using an actual measurement value (θ) of a rotation angle of the electric motor and a model correction gain so as to output said estimated value ( θ̃) of the rotation angle; a reference model in which it is assumed that no friction is generated in the power column and which calculates a target value ( θ̂) of the rotation angle of the power column; a feedback manipulated variable calculation unit that calculates a feedback manipulated variable by performing a feedback operation on a deviation between the target value of the rotation angle of the power column and the estimated value of the rotation angle of the power column; a motor torque command value calculation unit that calculates a motor torque command value by correcting the assist torque command value using the estimated value of the frictional force and the feedback manipulated variable; and a motor current control unit that controls a motor current flowing in the electric motor so that the motor torque of the electric motor becomes equal to the motor torque command value; wherein a feedback gain that is used in the feedback manipulated variable calculation unit and the model correction gain that is used in the observer are set so that a characteristic equation corresponding to an equation of motion representing behavior of the electric power steering system has a double root. a power column including an electric motor and a speed reduction mechanism that amplifies motor torque of the electric motor; an assist torque command value calculation unit that sets an assist torque command value of the electric motor; a plant observer that calculates an estimated value ( θ̃) of a rotation angle of the power column and an estimated value ( μ̃N ( d̃h)) of a frictional force that is generated in the power column, by using an observer model and a frictional force estimating unit which outputs said estimated value ( μ̃N ( d̃h)) of a frictional force, wherein in the observer model it is assumed that no friction is generated in the power column and whose characteristic is corrected by using an actual measurement value (θ) of a rotation angle of the electric motor and a model correction gain so as to output said estimated value ( θ̃) of the rotation angle; a reference model in which it is assumed that no friction is generated in the power column and which calculates a target value ( θ̂) of the rotation angle of the power column; a feedback manipulated variable calculation unit that calculates a feedback manipulated variable by performing a feedback operation on a deviation between the target value of the rotation angle of the power column and the estimated value of the rotation angle of the power column; a motor torque command value calculation unit that calculates a motor torque command value by correcting the assist torque command value using the estimated value of the frictional force and the feedback manipulated variable; and a motor current control unit that controls a motor current flowing in the electric motor so that the motor torque of the electric motor becomes equal to the motor torque command value; wherein a feedback gain that is used in the feedback manipulated variable calculation unit and the model correction gain that is used in the observer are set so that a characteristic equation corresponding to an equation of motion representing behavior of the electric power steering system has a double root.