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 .