An object of the present invention is to provide a method of enabling efficient structural analysis of a target protein that has been impossible or difficult to analyze so far, by stabilizing the target protein. The present invention provides an anti-BRIL antibody which specifically binds to BRIL or a BRIL fusion protein and an antigen-binding fragment thereof, a nucleic acid encoding the anti-BRIL antibody and the antigen-binding fragment thereof, a vector containing the nucleic acid, an antibody producing cell containing the vector, a method of producing the antibody, and a method of using the antibody in a protein structural analysis.