  A description of the nucleon and Roper resonance in a quark-diquark approach
is presented. We show that two states with the quantum number of the nucleon
can appear in the ground state of the spatial configuration, when there are two
types of diquarks: scalar-isoscalar and axial-vector-isovector diquarks. The
mass difference between the two states is generated by the mass difference
between the two diquarks, which is due to the spin-spin interaction between the
two quarks in the diquarks. The two states are then identified with the nucleon
and Roper resonance.
