Provided is a pressure steam treatment apparatus of a fiber bundle and a producing method of a carbon fiber precursor fiber bundle for producing a carbonated precursor fiber bundle to suppress an influence of a leakage of a pressure steam to the outside of the apparatus so as to hold down a pressure steam supply amount to the minimum, simultaneously reduce a broken yarn, improve a yield ratio and have a high productivity. A pressure steam treatment apparatus is provided with labyrinth sealing chambers in back and forth of a pressure steam treatment chamber, and treats a plurality of fiber bundles traveling in parallel like a sheet along a fiber bundle travel path in a lump under a pressure steam atmosphere. The fiber bundle travel path of the labyrinth sealing chamber is divided in parallel to the fiber bundle.