A thin film forming apparatus for forming a thin film on a substrate using plasma, comprising: an outside vacuum vessel constituting a first internal space; a bell jar provided in the first internal space of the outside vacuum vessel to constitute a second internal space; a plasma source provided in the first internal space, the plasma source receiving supply of a high-frequency power to generate the plasma; and a substrate stage provided opposite to the plasma source in the first internal space and configured in such a way that the substrate can be placed thereon in the second internal space, a first vacuum pump connected to the first internal space, and a second vacuum pump connected to the second internal space, a plasma source including plural antenna elements and provided in the first internal space, a feed unit configured to supply a power to each antenna element of the plasma source, the feed unit including a high-frequency signal generator and a distributor, and the high-frequency signal generator including an oscillator and an amplifier, wherein the second internal space is surrounded by the substrate stage and the bell jar, the first vacuum pump and the second vacuum pump are configured to control pressures of the first internal space and the second internal space and the feed unit is configured to control the power supplied to the plasma source such that the plasma is generated in the second internal space while not generated in the first internal space, the first vacuum pump and the second vacuum pump are configured to control the pressure of the second internal space and the pressure of the first internal space so as to have a pressure difference, the first vacuum pump and the second vacuum pump are configured to control the pressures of the first internal space and the second internal space such that the pressure of the first internal space becomes 1 × 10 -3 to 1 × 10 -2 Pa or 1200 to 3000 Pa and such that the pressure of the second internal space becomes 20 to 100 Pa, a antenna element constituting the plasma source, the bell jar, and the substrate stage are sequentially disposed from above in the outside vacuum vessel, and the outside vacuum vessel is separated into two regions (14a, 14b) such that the antenna element is separated from the bell jar and the substrate stage. an outside vacuum vessel constituting a first internal space; a bell jar provided in the first internal space of the outside vacuum vessel to constitute a second internal space; a plasma source provided in the first internal space, the plasma source receiving supply of a high-frequency power to generate the plasma; and a substrate stage provided opposite to the plasma source in the first internal space and configured in such a way that the substrate can be placed thereon in the second internal space, a first vacuum pump connected to the first internal space, and a second vacuum pump connected to the second internal space, a plasma source including plural antenna elements and provided in the first internal space, a feed unit configured to supply a power to each antenna element of the plasma source, the feed unit including a high-frequency signal generator and a distributor, and the high-frequency signal generator including an oscillator and an amplifier, the feed unit including a high-frequency signal generator and a distributor, and the high-frequency signal generator including an oscillator and an amplifier, wherein the second internal space is surrounded by the substrate stage and the bell jar, the first vacuum pump and the second vacuum pump are configured to control pressures of the first internal space and the second internal space and the feed unit is configured to control the power supplied to the plasma source such that the plasma is generated in the second internal space while not generated in the first internal space, the first vacuum pump and the second vacuum pump are configured to control the pressure of the second internal space and the pressure of the first internal space so as to have a pressure difference, the first vacuum pump and the second vacuum pump are configured to control the pressures of the first internal space and the second internal space such that the pressure of the first internal space becomes 1 × 10 -3 to 1 × 10 -2 Pa or 1200 to 3000 Pa and such that the pressure of the second internal space becomes 20 to 100 Pa, a antenna element constituting the plasma source, the bell jar, and the substrate stage are sequentially disposed from above in the outside vacuum vessel, and the outside vacuum vessel is separated into two regions (14a, 14b) such that the antenna element is separated from the bell jar and the substrate stage.