We prepared antibody-immobilized gel layers on surface plasmon resonance (SPR) sensor chips by atom transfer radical polymerization (ATRP). In a buffer solution with a target antigen, the SPR signal of the antibody-immobilized gel layer chip with an optimal thickness was much larger than that of a directly antibody-immobilized chip prepared by the standard amine coupling method. The antibody-immobilized gel layer chips are candidates as highly sensitive bioconjugated devices for SPR sensing systems. This paper focuses on the relationship between the thickness of the gel layer and the SPR signal in the detection of a target antigen.