PROBLEM TO BE SOLVED: To provide a deposition device capable of depositing a low dielectric constant film on a substrate to be processed having a large diameter, and a method for forming the low dielectric constant film.SOLUTION: A processing container S defines a space including a plasma generation chamber S1 and a processing chamber S2 at a lower side of the plasma generation chamber. A first gas supply system H1 supplies rare gas to the plasma generation chamber. A dielectric window 16 is provided to seal the plasma generation chamber. An antenna 14 supplies microwaves through the dielectric window to the plasma generation chamber. A second gas supply system H2 supplies precursor gas to the processing chamber. A shield part 40 is provided between the plasma generation chamber and the processing chamber, includes a plurality of openings 40h communicating the plasma generation chamber and the processing chamber, and has shieldability with respect to ultraviolet rays. In the deposition device, pressure in the plasma generation chamber is set to four times or more as high as pressure in the processing chamber, and a diffusion degree of the precursor gas from the processing chamber to the plasma generation chamber is set to 0.01 or less.