To detect 2,4,6-trinitrotoluene (TNT) using a displacement method over a surface plasmon resonance immunosensor, we developed sensor surfaces modified with a poly-(N-vinylformamide-co-vinylamine), which was synthesized from a monomer, N-vinylformamide, and was hydrolyzed to produce amine groups, and TNT analogs. Poly-(N-vinylformamide-co-vinylamine) that reacted with succinic acid was used as a matrix of a sensor surface for increasing the number of antibody binding sites. 2,4-Dinitrophenyl glycine, 3-nitrophenyl acetic acid, and 4-nitrophenyl acetic acid were immobilized on the polymer-modified sensor surfaces. We evaluated the suppression ability of the sensor surface for nonspecific adsorption and determined the limit of detection (LOD) for TNT. As a result, a sensor chip suppressed nonspecific adsorption for 1 mg/mL bovine serum albumin and lysozyme. TNT was measured by the displacement method, and the LOD achieved over a 3-nitrophenyl acetic acid-modified sensor chip was 0.9 ng/mL (ppb).