A method of controlling a series elastic actuator (SEA) comprising a motor-side rotation unit and load-side rotation unit coupled to transfer rotatory power of a driving motor to a load and elastic members and a sensor part comprising a magnetic body and a hall sensor , the method comprising: measuring relative displacement between the motor-side rotation unit and the load-side rotation unit; calculating external torque by an external force applied to the load side based on the measured displacement and hardness (K) of the elastic member; comparing the calculated external torque with a critical torque (τ th ); and switching a control mode of the SEA to any one of torque control and position control based on a result of the comparison, when the external torque increases to a value greater than the critical torque (τ th ), the control mode switches from the position control to the torque control, and when the external torque decreases to a value (τ th -τ hys ) obtained by subtracting given torque (τ hys ) from the critical torque (τ th ) or less, the control mode switches from the torque control to the position control, wherein an outer frame having accommodation spaces to which the pair of elastic members is fixed is formed in any one of the motor-side rotation unit and the load-side rotation unit, wherein an inner frame for supporting the pair of elastic members therein is formed in another of the motor-side rotation unit and the load-side rotation unit, wherein the displacement is measured using the magnetic body and the hall sensor respectively positioned at corresponding positions of the motor-side rotation unit and the load-side rotation unit, and wherein the magnetic body is positioned between the two inner frames adjacent to each other, and the hall sensor is positioned on the outer frame. measuring relative displacement between the motor-side rotation unit and the load-side rotation unit; calculating external torque by an external force applied to the load side based on the measured displacement and hardness (K) of the elastic member; comparing the calculated external torque with a critical torque (τ th ); and switching a control mode of the SEA to any one of torque control and position control based on a result of the comparison, when the external torque increases to a value greater than the critical torque (τ th ), the control mode switches from the position control to the torque control, and when the external torque decreases to a value (τ th -τ hys ) obtained by subtracting given torque (τ hys ) from the critical torque (τ th ) or less, the control mode switches from the torque control to the position control, wherein an outer frame having accommodation spaces to which the pair of elastic members is fixed is formed in any one of the motor-side rotation unit and the load-side rotation unit, wherein an inner frame for supporting the pair of elastic members therein is formed in another of the motor-side rotation unit and the load-side rotation unit, wherein the displacement is measured using the magnetic body and the hall sensor respectively positioned at corresponding positions of the motor-side rotation unit and the load-side rotation unit, and wherein the magnetic body is positioned between the two inner frames adjacent to each other, and the hall sensor is positioned on the outer frame. <EOS>