Provided is a carbonization furnace in which disordering of fiber bundles does not occur and there is no lack of uniformity throughout the entire furnace interior, even in the supply of heated inert gas. A carbonization furnace for manufacturing carbon fiber bundles, the furnace being provided with a heat treatment chamber, an inlet sealed chamber and an outlet sealed chamber, a gas spray nozzle, and a conveyance path, wherein: the gas spray nozzle (4) has a double tube structure obtained from a hollow cylindrical inner tube (8) and a hollow cylindrical outer tube (7), and is disposed in a direction that is horizontal and is orthogonal to the fiber bundle conveyance direction; in the outer tube, multiple gas-spraying holes (7a) are disposed across the width of the conveyance path in the longitudinal direction of the outer tube, and the area of the gas-spraying holes of the outer tube is 0.5 mm2 to 20 mm2; in the inner tube, multiple gas-spraying holes (8a) are disposed across the width of the conveyance path in the longitudinal direction of the inner tube such that the gas-spraying directions of the gas-spraying holes are in two or more directions of the circumferential direction of the inner tube, and the interval of the gas-spraying holes of the inner tube in the longitudinal direction of the inner tube is 300 mm or less.