A pressure steam treatment apparatus according to the invention includes a pressure steam treatment chamber and labyrinth sealing chambers. The labyrinth sealing chambers are respectively arranged on a fiber bundle inlet and on a fiber bundle outlet of the steam treatment apparatus, having a running path of the fiber bundle in a horizontal direction and having plural labyrinth nozzles on top and bottom of the running path. The difference between a maximum value and a minimum value of the distance in the perpendicular direction of the top and bottom side labyrinth nozzles, of a pair of opposing labyrinth nozzles is 0.5 mm or smaller when the ambient temperature of the labyrinth sealing chamber is 140° C. This structure ensures that the energy cost can be reduced, the deformation of the apparatus and also, the raise of fuzz on the fiber bundle and fiber bundle breakage can be prevented at the same time.