Abstract: A method in which a miscellaneous drain water purifying device having a plurality of separator tanks connected to miscellaneous drain water sources via a change valve, receiving miscellaneous drain water, leaving it stagnant, and separating sedimentation and floating substances and a decomposition tank connected to a plurality of the respective separator tanks and receiving flow-out liquid from the sedimentation separator tanks is used, while one of the sedimentation separator tanks receives miscellaneous drain water and carries out sedimentation separation of sludge, sends out the top level clear water in the quantity equal to the received water quantity to the decomposition tank and carries out purifying treatment, an aeration treatment of the sludge separated from miscellaneous drain water by the previous stage prior to transfer is carried out in the other separator tank which is shut off from the miscellaneous drain water sources via the change valve; such transfer is alternatively repeated at suitable in