To prevent a drain from being clogged with polishing chips or the like by connecting a tube to supply air from a compressed air source to a part just before a drain valve in the drain pipe arranged in a bottom part of a polishing processing vessel.

One end of an air tube 12 to always supply air in a drain pipe 8 is opened (13) just before an inflow part 11 in the drain pipe 8, that is, a drain valve 9. The other end of the air tube 12 is connected to a compressed air supply source 14 through an air valve. Since air is always sent into the inflow part 11 in the drain pipe 8 through the air tube 12 the polishing processing time, convection is generated in a muddy fluid in a polishing processing vessel 3 and the drain pipe 8, and therefore, even if the drain valve 9 is opened at any time, the drain pipe 8 is not clogged.