Objective: Autoimmune nodopathy (AN), a newly recognized disease entity, is an immune-mediated polyneuropathy involving autoantibodies against cell adhesion molecules located in nodes of Ranvier and paranodal regions, such as neurofascin 186 (NF186) and neurofascin 155 (NF155). The present study aimed to identify the epitopes for autoantibodies against NF155 in a large cohort of Japanese patients with anti-NF155 antibody-positive (anti-NF155+) AN. Methods: Human embryonic kidney 293 cells stably expressing NF155, NF186, or the third to fourth fibronectin type III domain region (Fn3-Fn4) of NF155, as well as cells transiently expressing Fn3, Fn4, or the shorter Fn3-Fn4 region of NF155, were developed. Western blotting and flow cytometric cell-based assay (CBA) analyses were performed to determine the expression levels of the proteins and identify their target epitopes in serum samples from 100 IgG4 anti-NF155+ patients, four non-IgG4 anti-NF155+ patients, and eight healthy controls. Results: The expression levels of NF186, NF155, Fn3-Fn4 of NF155, and the other truncation variants of NF155 were confirmed by western blotting and flow cytometric CBA. Flow cytometric CBA analysis showed that the autoantibodies in all 104 anti-NF155+ patients bound to Fn3-Fn4. No autoantibodies reacted with NF186, Fn4, or shorter Fn3-Fn4, although the autoantibodies in one IgG4 anti-NF155+ patient (1.0%) recognized Fn3 in addition to Fn3-Fn4. Western blotting analysis of representative samples generally reproduced the CBA results. Interpretation: The present study involving a large cohort of patients clarified that the primary epitope for anti-NF155 antibodies is located in the Fn3-Fn4 region, but not in the Fn3 or Fn4 domains alone.