To precisely detect the external force or external torque added to an electric motor, electric motor system, or electric robot without using a force or torque sensor nor having an influence by the presence of static frictional force by making the external force or external torque added to a body to be driven by the electric motor directly appear in the driving current of this motor.

In an electric motor 1 to which a body 2 to be driven is connected, the external force or external torque detecting device for electric motor system is formed out of a dither signal generator 14 for applying and superposing to the driving current of the electric motor 1, a dither signal having a fine amplitude of a frequency higher than the frequency of the driving current, a current detector for detecting the change of the driving current of the electric motor 1, and motor computing elements 15, 16, 17 for calculating the external force or external torque added to the electric motor 1 from the current change detected by the current detector 12.