  We study chiral partner structure of four light nucleons, $N(939)$,
$N(1440)$, $N(1535)$ and $N(1650)$ using an effective chiral model based on the
parity doublet structure. In our model we introduce four chiral
representations, $({\bf 1},{\bf 2})$, $({\bf 2},{\bf 1})$, $({\bf 2},{\bf 3})$
and $({\bf 3},{\bf 2})$ under ${\rm SU}(2)_{\rm L} \otimes {\rm SU}(2)_{\rm R}$
symmetry. We determine the model parameters by fitting them to available
experimental values of masses, widths and the axial charge of $N(939)$ together
with the axial charges of $N(1535)$ and $N(1650)$ by lattice analyses. We find
five groups of solutions: In a group the chiral partner to $N(939)$ is
$N(1440)$ having small chiral invariant mass. In another group, the chiral
partner is a mixture of $N(1535)$ and $N(1650)$ having a large chiral invariant
mass. We claim that off-diagonal elements of axial-charge matrix can be used
for distinguishing these groups. We also discuss changes of masses associated
with chiral symmetry restoration, which could emerge in high density matter.
