A fluid flow-path device for supplying target fluid, comprising: a flow-path formation body having multiple flow paths and a side surface, the multiple flow paths being formed inside the flow-path formation body and allowing a flow of the target fluid in the flow paths and an inlet of each of the multiple flow paths opening at the side surface; and a distribution header arranged on the side surface to cover the inlet of each of the multiple flow paths and forming a distribution space for distributing the target fluid to each of the multiple flow paths between the distribution header and the flow-path formation body, wherein the distribution header includes a header body having a recessed portion and a supply port, the recessed portion opening to the side surface to form the distribution space in a state in which the distribution header is arranged on the side surface and the supply port being communicated with the distribution space to allow supply of the target fluid to the distribution space through the supply port, and a partition member provided at the header body at a position in the distribution space, partitioning the distribution space into an upstream-side space (14S1) communicated with the supply port and a downstream-side space (14S2) communicated with each of the multiple flow paths at a position closer to the flow-path formation body than the upstream-side space is to, and having a fluid passable portion allowing the target fluid supplied into the space through the supply port to flow to the downstream-side space from the upstream-side space while preventing a foreign substance contained in the target fluid from flowing to the downstream-side space from the upstream-side space, the fluid flow path device being characterized in that the header body is formed with an introduction port and a discharge port, the introduction port is communicated with the downstream-side space such that washing fluid for discharging the foreign substance adhering to the fluid passable portion from an inside of the space to an outside of the distribution header is supplied to the downstream-side space through the introduction port and passes through the fluid passable portion in a direction from the downstream-side space toward the upstream-side space to remove the foreign substance adhering to the fluid passable portion from the fluid passable portion, and the discharge port is communicated with the upstream-side space such that the washing fluid containing the foreign substance removed from the fluid passable portion by passing through the fluid passable portion in the direction from the downstream-side space toward the upstream-side space is discharged to the outside of the distribution header from the upstream-side space through the discharge port. a flow-path formation body having multiple flow paths and a side surface, the multiple flow paths being formed inside the flow-path formation body and allowing a flow of the target fluid in the flow paths and an inlet of each of the multiple flow paths opening at the side surface; and a distribution header arranged on the side surface to cover the inlet of each of the multiple flow paths and forming a distribution space for distributing the target fluid to each of the multiple flow paths between the distribution header and the flow-path formation body, wherein the distribution header includes a header body having a recessed portion and a supply port, the recessed portion opening to the side surface to form the distribution space in a state in which the distribution header is arranged on the side surface and the supply port being communicated with the distribution space to allow supply of the target fluid to the distribution space through the supply port, and a partition member provided at the header body at a position in the distribution space, partitioning the distribution space into an upstream-side space (14S1) communicated with the supply port and a downstream-side space (14S2) communicated with each of the multiple flow paths at a position closer to the flow-path formation body than the upstream-side space is to, and having a fluid passable portion allowing the target fluid supplied into the space through the supply port to flow to the downstream-side space from the upstream-side space while preventing a foreign substance contained in the target fluid from flowing to the downstream-side space from the upstream-side space, the fluid flow path device being characterized in that the header body is formed with an introduction port and a discharge port, the introduction port is communicated with the downstream-side space such that washing fluid for discharging the foreign substance adhering to the fluid passable portion from an inside of the space to an outside of the distribution header is supplied to the downstream-side space through the introduction port and passes through the fluid passable portion in a direction from the downstream-side space toward the upstream-side space to remove the foreign substance adhering to the fluid passable portion from the fluid passable portion, and the discharge port is communicated with the upstream-side space such that the washing fluid containing the foreign substance removed from the fluid passable portion by passing through the fluid passable portion in the direction from the downstream-side space toward the upstream-side space is discharged to the outside of the distribution header from the upstream-side space through the discharge port.