To provide a horizontal steam separator for a steam drum capable of improving water separating performance at respective horizontal steam separators by equalizing the steam quantity introduced into respective horizontal steam separators, capable of preventing the generation of carry-over, capable of improving efficiency by suppressing a fall of steam temperature and also capable of avoiding undesirable influence on a turbine blade of a steam turbine.

A perforated plate 10 for controlling flow passage resistance is mounted at the inlet part of a steam inlet duct 8a of the horizontal steam separator 8.