  In-medium effects in photon excitation of the S$_{11}$ resonance from nucleon
and its decay to $\eta$-meson using the RMF approach is studied. We examine the
in-medium effects by varing the Dirac fields of the nucleon and the S$_{11}$
resonance separately. We obtained a conclusion that the Dirac fields of the
nucleon, which reduces the nucleon effective mass, enlarge the width of
S$_{11}$ resonance, and small Dirac-fields of the resonance shifts the peak
position, and these combined effects can explain the in-medium properties
observed in the experimental data for $^{12}$C($\gamma$,$\eta$) reaction.
