A saturator (20;30;60) comprising: a flow path through which a first fluid can flow; one or more first heat exchange units (22a-c) disposed along a flow direction in the flow path of the first fluid, the one or more first heat exchange units (22a-c) configured to cause heat exchange between the first fluid and a second fluid; a second heat exchange unit disposed on a downstream side of the one or more first heat exchange units (22a-c) along the flow direction in the flow path of the first fluid, the second heat exchange unit configured to cause heat exchange between a third fluid and the first fluid after passing through the one or more first heat exchange units (22a-c); a humidifying unit (26a-d) configured to add water to the first fluid flowing through the flow path on upstream sides, along the flow direction in the flow path of the first fluid, of the one or more first heat exchange units (22a-c) and second heat exchange unit; and a conveyance path configured to convey the third fluid after the third fluid has been heat-exchanged in the second heat exchange unit from the second heat exchange unit to the upstream side, along the flow direction in the flow path of the first fluid, of the one or more first heat exchange units (22a-c) so as to flow the third fluid into the flow path as the first fluid; wherein the configuration is such that, in operation, in the first heat exchange unit (s) (22a-c), the first fluid can be heated and humidified, and the second fluid can be cooled and discharged outside; and such that, in the second heat exchange unit, the first fluid can be heated and humidified to saturation vapor pressure, and discharged outside, and the third fluid can be cooled. a flow path through which a first fluid can flow; one or more first heat exchange units (22a-c) disposed along a flow direction in the flow path of the first fluid, the one or more first heat exchange units (22a-c) configured to cause heat exchange between the first fluid and a second fluid; a second heat exchange unit disposed on a downstream side of the one or more first heat exchange units (22a-c) along the flow direction in the flow path of the first fluid, the second heat exchange unit configured to cause heat exchange between a third fluid and the first fluid after passing through the one or more first heat exchange units (22a-c); a humidifying unit (26a-d) configured to add water to the first fluid flowing through the flow path on upstream sides, along the flow direction in the flow path of the first fluid, of the one or more first heat exchange units (22a-c) and second heat exchange unit; and a conveyance path configured to convey the third fluid after the third fluid has been heat-exchanged in the second heat exchange unit from the second heat exchange unit to the upstream side, along the flow direction in the flow path of the first fluid, of the one or more first heat exchange units (22a-c) so as to flow the third fluid into the flow path as the first fluid; wherein the configuration is such that, in operation, in the first heat exchange unit (s) (22a-c), the first fluid can be heated and humidified, and the second fluid can be cooled and discharged outside; and such that, in the second heat exchange unit, the first fluid can be heated and humidified to saturation vapor pressure, and discharged outside, and the third fluid can be cooled.