the upstream support plate 43 has substantially in the radially outer circumference a rotation stop groove 71 extending in the radial direction the downstream support plate 45 is a substantially rectangular cross section arc shaped component the downstream support plate 45 has substantially in the center of the downstream surface in the radial direction a recessed portion 75 in which a ring shaped protruding portion 73 provided at the lower part of the installation space 22 in the seal box 21 is to be fitted the downstream support plate 45 has in the vicinity of the outer circumference in the upstream surface in the radial direction a downstream retaining groove 77 for accommodating the downstream retainer 51 the width that is the radial length of the downstream retaining groove 77 is set a little larger than the total length of the downstream retainer 51 and the weld 53 the upstream surface of the downstream support plate 45 is cut off from the downstream retaining groove 77 to the inner circumferential end by a depth corresponding to the axial length of the brush seal bristles 39 so as to accommodate the brush seal bristles 39 the upstream support plate 43 and the downstream support plate 45 are butted against each other with the upstream retainer 49 accommodated in the upstream retaining groove 65 and the downstream retainer 51 accommodated in the downstream retaining groove 77 and with the restraint bristle retainer 57 accommodated in the restraint bristle retaining groove 69 and then the outer.