Screening of the autoantibodies and autoantigens which are specific to Sjogren's syndrome, particularly primary Sjogren's syndrome, and provision of a prophylactic and therapeutic drug for Sjogren's syndrome, particularly primary Sjogren's syndrome, and a highly sensitive diagnostic agent for Sjogren's syndrome. The .alpha.-fodrin or .alpha.-fodrin fragment of the invention is an antigen to the autoantibodies specifically found in Sjogren's syndrome and this autoantigen finds application as a prophylactic and therapeutic drug for autoimmune diseases, particularly Sjogren's syndrome, or a diagnostic agent for such diseases.