  We explore applications of the maximum entropy method (MEM) to determine
properties of unstable particles using the four-dimensional O(4) $\phi^4$
theory as a laboratory. The spectral function of the correlation function of
the unstable $\sigma$ particle is calculated with MEM, and shown to yield
reliable results for both the mass of $\sigma$ and the energy of the two-pion
state. Calculations are also made for the case in which $\sigma$ is stable.
Distinctive differences in the volume dependence of the $\sigma$ mass and
two-pion energy for the stable and unstable cases are analyzed in terms of
perturbation theory.
