A control apparatus for controlling an exciting current of a field winding type dynamo-electric machine having an exciting winding so as to obtain a demanded output thereof comprises an exciting current detecting circuit being configured to detect an exciting current of the field winding type dynamo-electric machine, a rotation speed detecting circuit being configured to detect a rotation speed of a rotor of the field winding type dynamo-electric machine, an output estimating circuit being configured to estimate an output of the field winding type dynamo-electric machine, an exciting current control circuit being configured to correct the exciting current of the field winding type dynamo-electric machine so as to obtain the demanded output and to supply the corrected exciting current to an exciting winding of the field winding type dynamo-electric machine considering a multi-valued functionality of the output of the field winding type dynamo-electric machine with respect to the exciting current.