An annular valve (50, 70), comprising: an annular valve seat including a seat portion formed by an outer circumferential edge and an inner circumferential edge of an annular opening; a rod configured to be movable along a direction orthogonal to the seat portion; an annular valve body disposed so as to face the annular valve seat and fixed to the rod; and a valve-body moving unit for moving the annular valve body between a valve-closed position in which the annular valve body contacts the seat portion of the annular valve seat so that the annular opening is closed by the annular valve body, and a valve-open position in which the annular valve body is positioned away from the seat portion of the annular valve seat so that the annular opening is open, wherein the annular valve body includes a first surface facing the annular valve seat and a second surface disposed opposite to the first surface, the first surface being flat and parallel to a radial direction of the annular opening, wherein the seat portion of the annular valve seat is configured to be in surface contact with the flat first surface of the annular valve body in the valve-closed position in a first annular region including the outer circumferential edge of the annular opening and in a second annular region including the inner circumferential edge of the annular opening, characterized in that the annular valve body further comprises a valve-body shroud portion disposed opposite to the annular valve seat across the annular valve body in an axial direction of the annular opening, the valve-body shroud portion being configured to cover the second surface and an outer circumferential surface of the annular-valve body at least partially in the valve-open position such that the annular valve body is restricted from being forced toward the annular valve seat by fluid flow passing through the annular valve (50, 70), wherein the valve-body shroud portion includes: a base portion having an annular shape and being disposed so as to face the second surface of the annular valve body; and a rim portion protruding toward the annular valve seat from an outer circumferential edge of the base portion, and wherein a distance (g) in a radial direction between an inner circumferential edge (62a1) of a tip end surface of the rim portion and the outer circumferential surface of the annular valve body increases toward the second surface from the first surface of the annular valve body. an annular valve seat including a seat portion formed by an outer circumferential edge and an inner circumferential edge of an annular opening; a rod configured to be movable along a direction orthogonal to the seat portion; an annular valve body disposed so as to face the annular valve seat and fixed to the rod; and a valve-body moving unit for moving the annular valve body between a valve-closed position in which the annular valve body contacts the seat portion of the annular valve seat so that the annular opening is closed by the annular valve body, and a valve-open position in which the annular valve body is positioned away from the seat portion of the annular valve seat so that the annular opening is open, a valve-closed position in which the annular valve body contacts the seat portion of the annular valve seat so that the annular opening is closed by the annular valve body, and a valve-open position in which the annular valve body is positioned away from the seat portion of the annular valve seat so that the annular opening is open, wherein the annular valve body includes a first surface facing the annular valve seat and a second surface disposed opposite to the first surface, the first surface being flat and parallel to a radial direction of the annular opening, wherein the seat portion of the annular valve seat is configured to be in surface contact with the flat first surface of the annular valve body in the valve-closed position in a first annular region including the outer circumferential edge of the annular opening and in a second annular region including the inner circumferential edge of the annular opening, characterized in that the annular valve body further comprises a valve-body shroud portion disposed opposite to the annular valve seat across the annular valve body in an axial direction of the annular opening, the valve-body shroud portion being configured to cover the second surface and an outer circumferential surface of the annular-valve body at least partially in the valve-open position such that the annular valve body is restricted from being forced toward the annular valve seat by fluid flow passing through the annular valve (50, 70), wherein the valve-body shroud portion includes: a base portion having an annular shape and being disposed so as to face the second surface of the annular valve body; and a rim portion protruding toward the annular valve seat from an outer circumferential edge of the base portion, and wherein a distance (g) in a radial direction between an inner circumferential edge (62a1) of a tip end surface of the rim portion and the outer circumferential surface of the annular valve body increases toward the second surface from the first surface of the annular valve body.