A microwave plasma CVD apparatus for forming a functional deposited film comprises a film-forming chamber having a discharge space, a substrate holder, and apparatus for introducing a film-forming raw material gas into the film-forming chamber, for transmitting microwaves into the film-forming chamber to apply microwave energy to the raw material gas so that the raw material gas is converted into plasma, and for simultaneously applying a bias voltage to the plasma generated in the discharge space from an external system to control the plasma potential. A mechanism is provided for temporarily suspending the application of the bias voltage in the case when abnormal discharge occurs, while monitoring the fluctuation of the bias voltage to be applied to the plasma, to thereby inhibit the occurrence of abnormal discharge.