In a control apparatus for an inverter generator having a first, second and third windings wound around an alternator driven by an engine, and first, second and third inverters adapted to convert alternating current outputted from the first, second and third windings into direct current and invert the converted direct current into single-phase alternating current in a desired frequency so as to supply to the electric load, there is provided with an actuator (stepper motor) to open and close a throttle valve of the engine, and it is configured to detect output powers (effective powers) of the first, second and third inverters, to determine a target engine speed based on the detected output powers, and to control an operation of the actuator so that the engine speed converges at the determined target engine speed.