  The relation between scattering and production amplitudes are investigated,
using a simple field theoretical model, from the general viewpoint of unitarity
and the applicability of final state interaction(FSI-) theorem. The IA-method
and VMW-method, which are applied to our phenomenological analyses suggesting
the \sigma-existence, are obtained as the physical state representations of
scattering and production amplitudes, respectively. Moreover, the VMW-method is
shown to be an effective method to obtain the resonance properties from general
production processes, while the conventional analyses based on the
``universality'' of \pi\pi-scattering amplitude are powerless for this purpose.
