Abstract: It is an object of the present invention to provide a method that can diagnose traumatic brain injury irrespective of its degree, even when it is particularly mild. Disclosed is a method for diagnosing traumatic brain injury, including the steps of: (1) bringing Glial Fibrillary Acidic Protein (GFAP) in a sample collected from a subject into contact with an anti-GFAP capture antibody to form a complex containing the GFAP and the anti-GFAP capture antibody, (2) obtaining a measured value of the GFAP by detecting the GFAP in the complex, and (3) determining whether the subject has traumatic brain injury based on the measured value, wherein the epitope of the anti-GFAP capture antibody is within the 60th to 383rd amino acid sequences in SEQ ID NO: 1.