  We make a re-analysis of I=0 \pi\pi scattering phase shift \delta_0^0 through
a new method of S-matrix parametrization (IA; interfering amplitude method),
and show a result suggesting strongly for the existence of \sigma-particle --
long-sought Chiral partner of \pi-meson. Furthermore, through the
phenomenological analyses of typical production processes of the 2\pi-system,
the pp-central collision and the J/\Psi \to \omega\pi\pi decay, by applying an
intuitive formula as sum of Breit-Wigner amplitudes, (VMW; variant mass and
width method), the other evidences for the \sigma-existence are given.
  The validity of the methods used in the above analyses is investigated, using
a simple field theoretical model, from the general viewpoint of unitarity and
the applicability of final state interaction(FSI-) theorem, especially in
relation to the ``universality'' argument. It is shown that the IA and VMW are
obtained as the physical state representations of scattering and production
amplitudes, respectively. The VMW is shown to be an effective method to obtain
the resonance properties from production processes, which generally have the
unknown strong-phases. The conventional analyses based on the ``universality''
seem to be powerless for this purpose.
