The non-labeled and direct immunoassay of cytokine on a thin polyion complex film of poly-L-lysine and poly-styrenesulfonate was studied using a surface plasmon resonance sensor. The suppression of non-specific protein adsorption was investigated by changing the blend ratio of poly-L-lysine and poly-styrenesulfonate. With our film, the protein adsorption fell to less than 10% of that of an unmodified surface when the poly-L-lysine to poly-styrenesulfonate ratio was 4:6. We also measured the kinetic rates between tumor necrosis factor-α and anti-tumor necrosis factor-α antibodies, which were immobilized on the film. The binding constant was calculated to be 1.5×108 (M-1), which satisfies the binding constant level for a monoclonal antibody modified on a commercially available surface.