A film-forming device includes: a microwave supplying unit, which supplies microwaves for generating plasma along a treatment surface of a central conductor comprising at least a conductive workpiece material; a negative voltage applying unit, which applies to the workpiece material a negative bias voltage for expanding a sheath layer along the treatment surface of the workpiece material; a microwave transmitting window, which make the microwave, which is supplied by the microwave supplying unit, propagate to the expanded sheath layer through a microwave transmitting surface thereof, and a surrounding wall, which surrounds the microwave transmitting surface of the microwave transmitting window and protrudes beyond the microwave transmitting surface in a propagation direction in which the microwaves propagate.