An energy exchange chamber for exchanging pressure energy between concentrated seawater discharged from a reverse-osmosis membrane-separation apparatus and a part of seawater to be treated by the reverse-osmosis membrane separation apparatus in a seawater desalination system for producing fresh water from the seawater by supplying the seawater to the reverse-osmosis membrane-separation apparatus to separate the seawater into fresh water and concentrated seawater, said energy exchange chamber comprising: a chamber having a space for containing concentrated seawater and seawater therein; a concentrated seawater port provided at a lower part of said chamber for supplying and discharging the concentrated seawater; a seawater port provided at an upper part of said chamber for supplying and discharging the seawater; a concentrated seawater distributor communicating with said concentrated seawater port and configured to distribute a flow of the concentrated seawater flowing therein all over a horizontal plane of an interior of said chamber; and a seawater distributor communicating with said seawater port and configured to distribute a flow of the seawater flowing therein all over a horizontal plane of the interior of said chamber; wherein the concentrated seawater and the seawater introduced into said chamber are brought into direct contact with each other all over the horizontal plane of the interior of said chamber to exchange pressure energy between the concentrated seawater and the seawater, characterized in that: said chamber comprises a cylindrical chamber whose longitudinal direction is arranged horizontally; said concentrated seawater distributor is horizontally arranged and comprises a pipe (26; 36) having a plurality of holes (26h; 36h) arranged at a lower part thereof so as to face the lowest part of the inner surface of said chamber, said plurality of holes (26h; 36h) penetrating a wall of said pipe (26; 36) into hollow portion of said pipe (26; 36); and said seawater distributor is horizontally arranged and comprises a pipe (25; 35) having a plurality of holes (25h; 35h) arranged at an upper part thereof so as to face the highest part of the inner surface of said chamber, said plurality of holes (25h; 35h) penetrating a wall of said pipe (25; 35) into hollow portion of said pipe (25; 35). a chamber having a space for containing concentrated seawater and seawater therein; a concentrated seawater port provided at a lower part of said chamber for supplying and discharging the concentrated seawater; a seawater port provided at an upper part of said chamber for supplying and discharging the seawater; a concentrated seawater distributor communicating with said concentrated seawater port and configured to distribute a flow of the concentrated seawater flowing therein all over a horizontal plane of an interior of said chamber; and a seawater distributor communicating with said seawater port and configured to distribute a flow of the seawater flowing therein all over a horizontal plane of the interior of said chamber; wherein the concentrated seawater and the seawater introduced into said chamber are brought into direct contact with each other all over the horizontal plane of the interior of said chamber to exchange pressure energy between the concentrated seawater and the seawater, characterized in that: said chamber comprises a cylindrical chamber whose longitudinal direction is arranged horizontally; said concentrated seawater distributor is horizontally arranged and comprises a pipe (26; 36) having a plurality of holes (26h; 36h) arranged at a lower part thereof so as to face the lowest part of the inner surface of said chamber, said plurality of holes (26h; 36h) penetrating a wall of said pipe (26; 36) into hollow portion of said pipe (26; 36); and said seawater distributor is horizontally arranged and comprises a pipe (25; 35) having a plurality of holes (25h; 35h) arranged at an upper part thereof so as to face the highest part of the inner surface of said chamber, said plurality of holes (25h; 35h) penetrating a wall of said pipe (25; 35) into hollow portion of said pipe (25; 35). said chamber comprises a cylindrical chamber whose longitudinal direction is arranged horizontally; said concentrated seawater distributor is horizontally arranged and comprises a pipe (26; 36) having a plurality of holes (26h; 36h) arranged at a lower part thereof so as to face the lowest part of the inner surface of said chamber, said plurality of holes (26h; 36h) penetrating a wall of said pipe (26; 36) into hollow portion of said pipe (26; 36); and said seawater distributor is horizontally arranged and comprises a pipe (25; 35) having a plurality of holes (25h; 35h) arranged at an upper part thereof so as to face the highest part of the inner surface of said chamber, said plurality of holes (25h; 35h) penetrating a wall of said pipe (25; 35) into hollow portion of said pipe (25; 35).