PURPOSE: To generate a hollow yarn membrane without dust adhesion and clogging in a filtered water discharging section and a hollow section of a hollow system by providing a cleaning cap equipped with a cleaning water feed tube and a drain tube connectable each other on the inlet side of a hollow yarn membrane vessel.

CONSTITUTION: When cleaning a hollow yarn membrane, a filter cartridge enclosing active carbon is removed and a cleaning cap 7 is installed at the inlet 8 of a hollow yarn membrane vessel 3, the end of a filtered water discharger tube 6 being closed with a cap 11. Under such condition, raw water is fed from a cleaning water feed tube 9, and sprayed to hollow yarns 1, and dust adhered to the surface of hollow yarns 1 is washed away by water flow, the water being drained out of a drain tube 10.