A method for manufacturing a stabilized fiber bundle, the method comprising the step of: stabilizing a polyacrylonitrile precursor fiber bundle in an oxidizing atmosphere at 200 to 300°C to manufacture a stabilized fiber bundle, wherein in the stabilizing step, a fiber bundle is made to run, with respect to a roller group including m pieces (where m is an integer of 3 or more) of rollers arranged continuously, to sequentially pass between an n-th roller and an (n + 1)-th roller (where n is an integer of 1 or more and (m - 1) or less), the m pieces of rollers arranged continuously have roller axes parallel to each other and perpendicular to a running direction of the fiber bundle, the rollers have a roller diameter of 5 to 30 mm, and the fiber bundle has a specific gravity of 1.20 to 1.50, measured according to the method described in JIS R7601 and as described herein, and the method satisfies all of conditions (a) to (d) below: (a) L n satisfies 0.75 × (R n + R n + 1) ≤ L n ≤ 2.0 × (R n + R n + 1), wherein R n [mm] is a roller diameter of an n-th roller, R n + 1 [mm] is a roller diameter of an (n + 1)-th roller, and L n [mm] is a distance between an n-th roller axis and an (n + 1)-th roller axis; (b) a width W 0 of the fiber bundle before coming into contact with a first roller is in a range of 2.0 × 10 -4 to 6.0 × 10 -4 mm/dtex; (c) a width W 2 of the fiber bundle after leaving an m-th roller satisfies 1.0 ≤ W 2 /W 0 ≤ 1.1; and (d) a width W 1 of the fiber bundle on second to (m - 1)-th rollers satisfies W 1 /W 0 ≥ 1.4 in all the second to (m - 1)-th rollers, wherein the widths W 0, W 1 and W 2 of the fiber bundle were measured with the naked eye using a first grade stainless steel metal straight ruler specified in JIS B7516. stabilizing a polyacrylonitrile precursor fiber bundle in an oxidizing atmosphere at 200 to 300°C to manufacture a stabilized fiber bundle, wherein in the stabilizing step, a fiber bundle is made to run, with respect to a roller group including m pieces (where m is an integer of 3 or more) of rollers arranged continuously, to sequentially pass between an n-th roller and an (n + 1)-th roller (where n is an integer of 1 or more and (m - 1) or less), the m pieces of rollers arranged continuously have roller axes parallel to each other and perpendicular to a running direction of the fiber bundle, the rollers have a roller diameter of 5 to 30 mm, and the fiber bundle has a specific gravity of 1.20 to 1.50, measured according to the method described in JIS R7601 and as described herein, and the method satisfies all of conditions (a) to (d) below: (a) L n satisfies 0.75 × (R n + R n + 1) ≤ L n ≤ 2.0 × (R n + R n + 1), wherein R n [mm] is a roller diameter of an n-th roller, R n + 1 [mm] is a roller diameter of an (n + 1)-th roller, and L n [mm] is a distance between an n-th roller axis and an (n + 1)-th roller axis; (b) a width W 0 of the fiber bundle before coming into contact with a first roller is in a range of 2.0 × 10 -4 to 6.0 × 10 -4 mm/dtex; (c) a width W 2 of the fiber bundle after leaving an m-th roller satisfies 1.0 ≤ W 2 /W 0 ≤ 1.1; and (d) a width W 1 of the fiber bundle on second to (m - 1)-th rollers satisfies W 1 /W 0 ≥ 1.4 in all the second to (m - 1)-th rollers, (a) L n satisfies 0.75 × (R n + R n + 1) ≤ L n ≤ 2.0 × (R n + R n + 1), wherein R n [mm] is a roller diameter of an n-th roller, R n + 1 [mm] is a roller diameter of an (n + 1)-th roller, and L n [mm] is a distance between an n-th roller axis and an (n + 1)-th roller axis; (b) a width W 0 of the fiber bundle before coming into contact with a first roller is in a range of 2.0 × 10 -4 to 6.0 × 10 -4 mm/dtex; (c) a width W 2 of the fiber bundle after leaving an m-th roller satisfies 1.0 ≤ W 2 /W 0 ≤ 1.1; and (d) a width W 1 of the fiber bundle on second to (m - 1)-th rollers satisfies W 1 /W 0 ≥ 1.4 in all the second to (m - 1)-th rollers, wherein the widths W 0, W 1 and W 2 of the fiber bundle were measured with the naked eye using a first grade stainless steel metal straight ruler specified in JIS B7516.