A leaf seal device comprises a leaf seal having a plurality of sealing plates of thin plate shape, each of said sealing plates including a connection holding section of an outer circumferential side and a sealing section of an inner circumferential side, said plurality of sealing plates being overlapped along a circumferential direction by connecting said connection holding sections to form the leaf seal with an annular shape to which a rotary axis is available to fit at a center thereof, said leaf seal device sealing a sealed fluid by partitioning between an inner circumferential face of a through hole of a casing and said rotary axis which passes through the through hole, characterized in that said connection holding section is formed at an outer circumference side end portion of said sealing plate, said leaf seal device further comprises a back plate arranged on a lower pressure side of said leaf seal, said back plate comprising a fixing portion to support said connection holding section and a supporting face to support said leaf seal so that said leaf seal is not bent by a pressure of the sealed fluid, and a holding plate arranged on a high pressure side of said leaf seal, said holding plate sandwiching said leaf seal with the back plate (41 ( to support said leaf seal, said connection holding section is integrally associated with said fixing portion of said back plate and said holding plate to form the fixing portion for equipping said leaf seal device to said inner circumferential face of said through hole of said casing, said respective sealing plates are bent at a boundary between said connection holding section and said sealing section so that said connection holding section and said sealing section are crossed at a predetermined angle, said connection holding section of said respective sealing plates are connected by sequentially overlapping with bending status at a predetermined angle θ2 to a circumferential direction of said leaf seal and at another predetermined angle to a radial direction of said leaf seal, said sealing section of said respective sealing plates are provided so as to incline with a predetermined angle θ1 to said circumferential direction from said boundary with said connection holding section toward the inner circumferential side, said connection holding section of said respective sealing plates are formed as a flat portion without any bending portion, and a space of the sealing plates has a desired length based on a following formula: G = P × sin θ ⁢ 1 - t wherein G is the space between the respective sealing plates, P is a pitch of the sealing plates on a circumferential line defined by P=t/sin θ2, t is a plate thickness of the sealing plate, θ1 is an angle of a face of the sealing section to the circumferential line, and θ2 is an angle of a face of an inner circumferential side of the connection holding section to the circumferential line. G is the space between the respective sealing plates, P is a pitch of the sealing plates on a circumferential line defined by P=t/sin θ2, t is a plate thickness of the sealing plate, θ1 is an angle of a face of the sealing section to the circumferential line, and θ2 is an angle of a face of an inner circumferential side of the connection holding section to the circumferential line.