To provide the membrane separator arranging structure in a septic tank easy to perform the maintenance of a membrane separator.

In the arranging structure of a membrane separator 20 comprising a plurality of filter membranes provided in the tank body 11a of a septic tank 10a, guides 22, 21 assembled along the body peripheral surface of the membrane separator 20 are provided in the tank body 11a of the septic tank 10a so as to leave an interval up and down and a cavity 113a having the same shape as the bottom part of the membrane separator 20 is provided to the anchoring bottom part on which the membrane separator 20 is arranged of the tank body 11a of the septic tank 10a and the membrane separator 20 is drawn up along the upper and lower guides 21, 22.