To keep accuracy of a gap in a slit by increasing rigidity of a cover part in which gas injection slit is formed.
This nozzle box includes a cover part 22 in which left and right gas injection slits 29, 29 are formed, and a base part 23 in which a gas introduction hole 35 is opened. The cover part 22 is adapted such that left and right overlapped flanges 30, 30 by overlapping and joinning indivisibly outward bent flanges 27b, 27b extneding from left and right side plate parts 27, 27 on left and right outward bent flanges 28b, 28b extending from left and right vertical stand plate parts 28a, 28a of an intermediate part 28. Further, vent holes 28d, 28d... are drilled in the left and right outward bent flanges 28b, 28b of the intermediate parts 28, 28, and the base part 23 forms left and right outward bent flanges 32b, 32b extending longitudinally, and the left and right overlapped flanges 30, 30 and the outward bent flanges 32b, 32b of the base part 32 are joinned separably.