Performance of a moisture separator and a steam turbine plant is improved without increase in the size and the manufacturing cost thereof, by provision therein of: a body (31) that has a hollow shape and extends in a horizontal direction; a steam inlet portion (32) that is provided in a longitudinally intermediate portion of the body (31); a manifold (35, 36) that is arranged with a predetermined gap from an inner wall surface of the body (31), has a longitudinally intermediate portion communicating with the steam inlet portion (32), and has plural steam blowout portions (53, 54) provided along a longitudinal direction; moisture separating elements (37, 38) that separate moisture from steam blown out from the plural steam blowout portions (53, 54); and steam outlet portions (33, 34) which are provided in the body (31) and through which the steam is discharged, the steam having had the moisture separated therefrom and flowing along an inner wall of the body (31).