There is provided a film separation device 1 equipped with a treatment tank 2 receiving a liquid to be treated, a film unit 3 to be immersed in the liquid to be treated, and a guide mechanism 4. The guide mechanism 4 is composed of holding sections 14 and a guide section 15. The holding sections 14 are provided on a side section of a film cassette 5 constituting the film unit 3, and the guide section 15 is fixed to the treatment tank 2. The guide section 15 has a tank-wall fixed section 15a, an upper extension section 15b, a held section 15c, a lower extension section 15d, and a tank-bottom fixed section 15e. When fixing the film unit 3 to the treatment tank 2, the holding sections 14 are fitted to the held section 15c, thereby guiding the film unit 3 to a predetermined position in the treatment tank 2.