A handheld engine-driven working machine comprises an internal combustion engine with a throttle valve, a throttle adjusting device for adjusting an opening degree of the throttle valve of the internal combustion engine, and a control device provided in the internal combustion engine. The control device is configured to detect a rotational speed and an amount of change in the rotational speed at every at least one rotation of the internal combustion engine. The control device determines that the throttle valve is partially opened when the amount of change in the rotational speed is greater than a predetermined value.