A steam separator is provided with a barrel (2), a swirler (8), a pick-off ring (1), a steam collecting pipe (29) and transverse pipes (11). A two-phase flow of steam and water ascends inside the barrel (2). The two-phase flow has a swirl force imparted by the swirler (8). Flow (43) near the inner surface of the barrel (2) is separated by the pick-off ring (1), etc. from flow (4) near the center of the barrel (2) and is guided so as to descend along the outer surface of the barrel (2). A major part of the flow (4) near the center of the barrel (2) is separated from the outside flow by the steam collecting pipe (29) and guided to the outside of the barrel (2) through the transverse pipes (11).