To completely recover balls for cleaning a condenser by reducing a flowing resistance.

A cage-like collector 1 in which a screen 2 is a vertical direction is mounted at a downward end of a discharge tube 53 connected to a downstream side of a condenser 50. A recovery pipe 8 is connected to a bottom of the collector 1, and a pump 10 is provided at the pipe 8. Most of balls 56 are captured by a screen 58, and fed to the condenser 50 by a pump 55. The balls 56 which have passed through the screen 58 are advanced into the collector 1, effectively accumulated at the bottom of the collector 1 without passing the screen 2, sucked to the pump 10. Thus, sea water is drained to the open sea via the screen 2.