According to a torque detecting method, gravitational torque applied to a rotary shaft of an arm is detected in a condition where position control of the arm is stopped, when the arm is located at a first position at which the arm is oriented in a direction different from a direction of gravitational force. Then, a gravity coefficient used for calculating gravitational torque corresponding to a position of the arm is calculated, based on the gravitational torque and the first position. Then, gravitational torque during position control is calculated, based on the position of the arm detected during the position control, and the gravity coefficient. Further, an actual output torque during the position control is calculated, based on operating torque applied to the rotary shaft and detected during the position control, and the gravitational torque calculated during the position control.