A carbonization furnace for manufacturing a carbon fiber bundle comprising: a heat treatment chamber for heating a fiber bundle which has a fiber bundle inlet and a fiber bundle outlet through which the fiber bundle is introduced and withdrawn and is filled with an inert gas; an inlet sealing chamber and an outlet sealing chamber for sealing the gas in the heat treatment chamber which are arranged to be adjacent to the fiber bundle inlet and the fiber bundle outlet of the heat treatment chamber, respectively; a gas jetting nozzle provided on at least one of the inlet sealing chamber and the outlet sealing chamber; and a conveying path for conveying the fiber bundle which is provided in the horizontal direction in the inlet sealing chamber, the heat treatment chamber, and the outlet sealing chamber, wherein the gas jetting nozzle has a double tube structure consisting of a hollow tubular inner tube and a hollow tubular outer tube and is disposed in a direction orthogonal and horizontal to a conveying direction of the fiber bundle, wherein a plurality of gas jetting holes are disposed on the outer tube in a longitudinal direction of the outer tube over the length corresponding to a width of the conveying path, and a hole area of the gas jetting holes of the outer tube is 0.5 mm 2 or more and 20 mm 2 or less, and a plurality of gas jetting holes are arranged on the inner tube in a longitudinal direction of the inner tube over the length corresponding to a width of the conveying path and a gas jetting direction of the gas jetting holes is arranged in two or more directions of a circumferential direction of the inner tube, and a hole interval between the gas jetting holes of the inner tube in the longitudinal direction of the inner tube is 300 mm or less. a heat treatment chamber for heating a fiber bundle which has a fiber bundle inlet and a fiber bundle outlet through which the fiber bundle is introduced and withdrawn and is filled with an inert gas; an inlet sealing chamber and an outlet sealing chamber for sealing the gas in the heat treatment chamber which are arranged to be adjacent to the fiber bundle inlet and the fiber bundle outlet of the heat treatment chamber, respectively; a gas jetting nozzle provided on at least one of the inlet sealing chamber and the outlet sealing chamber; and a conveying path for conveying the fiber bundle which is provided in the horizontal direction in the inlet sealing chamber, the heat treatment chamber, and the outlet sealing chamber, wherein the gas jetting nozzle has a double tube structure consisting of a hollow tubular inner tube and a hollow tubular outer tube and is disposed in a direction orthogonal and horizontal to a conveying direction of the fiber bundle, wherein a plurality of gas jetting holes are disposed on the outer tube in a longitudinal direction of the outer tube over the length corresponding to a width of the conveying path, and a hole area of the gas jetting holes of the outer tube is 0.5 mm 2 or more and 20 mm 2 or less, and a plurality of gas jetting holes are arranged on the inner tube in a longitudinal direction of the inner tube over the length corresponding to a width of the conveying path and a gas jetting direction of the gas jetting holes is arranged in two or more directions of a circumferential direction of the inner tube, and a hole interval between the gas jetting holes of the inner tube in the longitudinal direction of the inner tube is 300 mm or less.