A film deposition device in which film deposition is performed on an elongated substrate which is traveling in a longitudinal direction, the film deposition device comprising: a CVD film forming room disposed on a travel path of said substrate, including a material gas supply means and a first evacuation means and having a function of performing the film deposition on said substrate by CVD; a treatment room (14, 24) disposed upstream or downstream of said CVD film forming room on the travel path, including a second evacuation means (46, 78) and having a function of performing a predetermined treatment on said substrate; a differential room (16, 20) disposed between and communicating with said CVD film forming room and said treatment room (14, 24) on said travel path; and a drum on an outer peripheral surface of which said substrate travels, and wherein said CVD film forming room (18, 24), said film deposition treatment room (18, 24) and said differential room (16, 20, 26) form a room with the outer peripheral surface of said drum, wherein said drum serves as a counter electrode of a shower head electrode in said CVD film forming room, wherein said differential room (16, 20) includes a third evacuation means (52, 68), a gas introducing means (50, 64) for introducing at least one of a gas to be supplied to both of said CVD film forming room and said treatment room (14, 24), and an inert gas, wherein said differential room (16, 20) includes a control means (54, 70) which controls said third evacuation means (52, 68) and said gas introducing means (50, 64) to keep said differential room (16, 20) at a higher pressure than said CVD film forming room and said treatment room (14, 24). a CVD film forming room disposed on a travel path of said substrate, including a material gas supply means and a first evacuation means and having a function of performing the film deposition on said substrate by CVD; a treatment room (14, 24) disposed upstream or downstream of said CVD film forming room on the travel path, including a second evacuation means (46, 78) and having a function of performing a predetermined treatment on said substrate; a differential room (16, 20) disposed between and communicating with said CVD film forming room and said treatment room (14, 24) on said travel path; and a drum on an outer peripheral surface of which said substrate travels, and wherein said CVD film forming room (18, 24), said film deposition treatment room (18, 24) and said differential room (16, 20, 26) form a room with the outer peripheral surface of said drum, wherein said drum serves as a counter electrode of a shower head electrode in said CVD film forming room, wherein said differential room (16, 20) includes a third evacuation means (52, 68), a gas introducing means (50, 64) for introducing at least one of a gas to be supplied to both of said CVD film forming room and said treatment room (14, 24), and an inert gas, wherein said differential room (16, 20) includes a control means (54, 70) which controls said third evacuation means (52, 68) and said gas introducing means (50, 64) to keep said differential room (16, 20) at a higher pressure than said CVD film forming room and said treatment room (14, 24).