Method for manufacturing acrylonitrilic fiber bundle and method for manufacturing carbon fiber bundle

The present invention addresses the problem of providing a stretching method having superior process passability when subjecting an acrylonitrilic fiber bundle to pressurized steam stretching, in particular, when treatment is performed with a high degree of stretching or at high speed. The present invention is a method for manufacturing an acrylonitrilic fiber bundle, in which a fiber bundle made of threads prepared by spinning from a spinning solution containing an acrylonitrilic copolymer is subjected to pressurized steam stretching under a pressurized steam atmosphere by using a pressurized steam stretching device A at least having two regions, namely, a preheating region (1) provided on a fiber-bundle introducing side and a heat stretching region (2) provided on a fiber-bundle exit side, these two regions being separated by a sealing region (3B) provided with a sealing member. When the pressure in the preheating region (1) is signified by P1 (MPa) and the pressure in the heat stretching region (2) is sign