An anti-telescoping plate, configured to be provided to one of both end portions of a spirally wound body and an outer shell of a fluid separation element, the fluid separation element having an axial direction and comprising: the spirally wound body comprising a separation membrane (3, 4) and a raw water channel material that are spirally wound around a water collection tube; the outer shell provided on an outer circumference of the spirally wound body; and the anti-telescoping plate, wherein the anti-telescoping plate includes at least a first annular portion capable of retaining the outer shell, and a second annular portion which is formed on an axially outer side of the first annular portion, in which an outer end portion of the outer shell can be locked therein, and which has an annular shape with a larger outer diameter than an outer diameter of the first annular portion, wherein the first and the second annular portions (19, 20) have side surfaces on the axially inner and axially outer sides, the axially inner and axially outer sides being the side surfaces on the spirally wound body side and the side opposite thereto, respectively, wherein the anti-telescoping plate has at least one depression provided on an axially inner side surface of the second annular portion, wherein a part of the outer shell is capable of entering the at least one depression to prevent the outer shell from rotating relatively to each of the anti-telescoping plates, when the fluid separation element is assembled, and wherein sectional shapes or positions of the depression appearing when the second annular portion is sectionally cut by arbitrary planes passing the central axis of the anti-telescoping plate differ depending on the planes in a circumferential direction of the second annular portion. the spirally wound body comprising a separation membrane (3, 4) and a raw water channel material that are spirally wound around a water collection tube; the outer shell provided on an outer circumference of the spirally wound body; and the anti-telescoping plate, wherein the anti-telescoping plate includes at least a first annular portion capable of retaining the outer shell, and a second annular portion which is formed on an axially outer side of the first annular portion, in which an outer end portion of the outer shell can be locked therein, and which has an annular shape with a larger outer diameter than an outer diameter of the first annular portion, wherein the first and the second annular portions (19, 20) have side surfaces on the axially inner and axially outer sides, the axially inner and axially outer sides being the side surfaces on the spirally wound body side and the side opposite thereto, respectively, wherein the anti-telescoping plate has at least one depression provided on an axially inner side surface of the second annular portion, wherein a part of the outer shell is capable of entering the at least one depression to prevent the outer shell from rotating relatively to each of the anti-telescoping plates, when the fluid separation element is assembled, and wherein sectional shapes or positions of the depression appearing when the second annular portion is sectionally cut by arbitrary planes passing the central axis of the anti-telescoping plate differ depending on the planes in a circumferential direction of the second annular portion.