A film forming device includes: a microwave supplying unit configured to supply microwaves for generating plasma along a treatment surface of a conductive workpiece; a negative voltage applying unit configured to apply to the workpiece a negative bias voltage for expanding a sheath layer thickness along the treatment surface of the workpiece, and a controller configured to control the microwave supplying unit and the negative voltage applying unit, wherein the microwave supplying unit has a microwave transmitting window configured to propagate the supplied microwaves to the expanded sheath layer, wherein the controller is configured to control the microwave supplying unit and the negative voltage applying unit while supplying of the microwaves so that a sheath thickness of the sheath layer changes.