PURPOSE: To produce flame-resistant acrylonitrile fibers in a state substantially not containing broken fibers in the bundle of the fibers, by imparting a specific collecting agent to the acrylonitrile fiber bundle, drying the bundle at a specific temperature and further subjected the bundle to a flame-resistant treatment.
CONSTITUTION: An acrylonitrile fiber bundle is coated with a collecting agent comprising a water-soluble polymeric compound such as starch or polyacrylamide, dried at a temperature of ≤200°C, and subsequently subjected to a flame resistance treatment in a hot air-circulating oven, etc., to produce the objective flame-resistant fiber bundle reduced in the breakage of single filaments and in defects and having good physical properties and an appearance. The fiber bundle is preferably coated with a collecting agent comprising the water-soluble polymeric compound and a water-soluble compound ≥50% of which perfectly decomposes or sublimes at a temperature of ≤200°C.