  We study the pion-nucleon P_33 and P_11 scattering, where the Delta and the
Roper resonances are seen, respectively. We use the Lippmann-Schwinger equation
extended to couple to a one-body state, and investigate nature of these
resonances by taking and omitting a one-baryon state into account. We see
validity and puzzle of the standard quark model interpretation, for the Delta
and the Roper resonances, respectively.
