To reuse a raw water seal member and to simplify work for replacing a fluid separation element by forming a membrane unit including a separation membrane, a filtrate channel material, and a raw liquid channel material around a drainage pipe and arranging the raw water seal member in the end part of the membrane unit with an opening part turned upstream.

In a fluid separation element 1, a membrane unit 7 including a separation membrane 4, a filtrate channel material 5, and a raw liquid channel material 6 is installed around a drainage pipe 3 having drainage holes 2, a seal holder 30 for forming a raw liquid seal member 9 is installed on the upstream side in the raw liquid flow direction of the membrane unit 7. The member 9 having a C-shaped cross section is fitted freely attachably/detachably to the holder 30 so that its opening part is the upstream side. The element 1 is housed in a pressure container and used as a fluid separation membrane module. When a plurality of the elements 1 are housed, a telescope preventive plate 36 is formed in each downstream side end part to prevent the deformation of the membrane unit 7.