  We study the ground and first excited states of nucleons in a chiral
quark-diquark model. We include two quark-diquark channels of the
scalar-isoscalar and axial-vector-isovector types for the nucleon states. The
diquark correlation violating the spin-flavor SU(4)$_{SF}$ symmetry allows to
treat the two quark-diquark channels independently. Hence the two states appear
as the superpositions of the two quark-diquark channels; one is the nucleon and
the other is a state which does not appear in the SU(4)$_{SF}$ quark models.
With a reasonable choice of model parameters, the mass of the excited state
appears at around 1.5 GeV, which we identify with the Roper resonance N(1440).
