A pressure exchanger for a reverse osmosis membrane device for a seawater desalination facility for exchanging pressure between a first fluid and a second fluid, the pressure exchanger comprising: a rotator having, around a rotation axis, a pressure transmission section which is configured to enable the pressure exchange between the first fluid and the second fluid, characterised in that the pressure exchanger is provided such that a first flow path into or out of which the first fluid flows from a first end is communicated with a second flow path into or out of which the second fluid flows from the first end; wherein the first fluid supplied to the first fluid inflow path corresponds to high pressure concentrated seawater discharged from a reverse osmosis membrane device, and the second fluid supplied to the second fluid inflow path corresponds to low pressure seawater supplied to the reverse osmosis membrane device, the pressure exchanger further comprises: a first lateral member provided, in a thickness direction, with a first fluid inflow path for guiding the first fluid to the first flow path, a second fluid outflow path for guiding the second fluid after pressure exchange with the first fluid from the second flow path, a second fluid inflow path for guiding the second fluid to the second flow path, and a first fluid outflow path for guiding the first fluid after pressure exchange with the second fluid from the first flow path; and a second lateral member rotatably sandwiching the rotator between the first lateral member and the second lateral member, with a retentive member being interposed therebetween, wherein the first fluid inflow and outflow paths (14, 17) and the second fluid inflow and outflow paths (16, 15) are only provided in the first lateral member. a rotator having, around a rotation axis, a pressure transmission section which is configured to enable the pressure exchange between the first fluid and the second fluid, characterised in that the pressure exchanger is provided such that a first flow path into or out of which the first fluid flows from a first end is communicated with a second flow path into or out of which the second fluid flows from the first end; wherein the first fluid supplied to the first fluid inflow path corresponds to high pressure concentrated seawater discharged from a reverse osmosis membrane device, and the second fluid supplied to the second fluid inflow path corresponds to low pressure seawater supplied to the reverse osmosis membrane device, the pressure exchanger further comprises: a first lateral member provided, in a thickness direction, with a first fluid inflow path for guiding the first fluid to the first flow path, a second fluid outflow path for guiding the second fluid after pressure exchange with the first fluid from the second flow path, a second fluid inflow path for guiding the second fluid to the second flow path, and a first fluid outflow path for guiding the first fluid after pressure exchange with the second fluid from the first flow path; and a second lateral member rotatably sandwiching the rotator between the first lateral member and the second lateral member, with a retentive member being interposed therebetween, wherein the first fluid inflow and outflow paths (14, 17) and the second fluid inflow and outflow paths (16, 15) are only provided in the first lateral member. a first lateral member provided, in a thickness direction, with a first fluid inflow path for guiding the first fluid to the first flow path, a second fluid outflow path for guiding the second fluid after pressure exchange with the first fluid from the second flow path, a second fluid inflow path for guiding the second fluid to the second flow path, and a first fluid outflow path for guiding the first fluid after pressure exchange with the second fluid from the first flow path; and a second lateral member rotatably sandwiching the rotator between the first lateral member and the second lateral member, with a retentive member being interposed therebetween, wherein the first fluid inflow and outflow paths (14, 17) and the second fluid inflow and outflow paths (16, 15) are only provided in the first lateral member.