To provide a filter for cleaning drain water required for purified water by certainly oil/water separating air/liquid separation of drain containing air discharged from a compressor, or the like.

The filter 1 for cleaning drain water comprises an air/liquid separation part, an air breather 2 and a net member for removing a floating substance in the drain freed from air and capturing an oil component. The air breather 2 comprises a thin net-like body 13 for separating air from the drain by relaxing introduction speed of introduced air and that of the drain; a closed cylinder 4 for feeding the drain to a tray 6 side by releasing the air drain from a second through hole 11 of a lower plate 7 and releasing the air from an opening hole 10 of an upper lid 29 of a first tank 26; and the tray 6, or the like, having an another second through hole for dropping off the drain into the first tank 26 and carrying out gravity oil/water separation. Regarding the mixed solution oil/water-separated in the first tank 26, a part of the floating substance and the oil component is removed by the net member in the another second tank at the subsequent step. Thereby, the mixed solution containing a small amount of the oil component can be fed to the subsequent step side.