A first fixing member which fixes to a membrane module unit a tubular member with a protrusion formed at a tip end or near the tip end for connecting a membrane module of the membrane module unit and a water collecting header to form a filtrate flow path from the membrane module to the water collecting header, the first fixing member includes: a fixing member main body which includes an engaging surface formed at a position at which the fixing member main body engages with the protrusion formed in the tubular member; and a second fixing member which fixes the fixing member main body to the membrane module unit.