  We test application of the maximum entropy method to decompose the states
contributing to the unstable $\sigma$ correlation function through the spectral
function in the four dimensional O(4) $\phi^4$ theory. Reliable results are
obtained for the $\sigma$ mass and two-particle $\pi\pi$ state energy using
only the $\sigma$ correlation function. We also find that the property of the
$\sigma$ particle is different between the unstable ($m_{\sigma}/m_{\pi}>2$)
and stable ($m_{\sigma}/m_{\pi}<2$) cases.
