  In neutral kaon system, we always use non-hermitian Hamiltonian for
convenience of treating decay process, unitarity seems to be lost. If we take
decay channels (\pi\pi, \pi\pi\pi, \pi\ell\nu etc.) into account, however,
Hamiltonian of the whole system must be hermitian. We attempt to derive an
effective Hamiltonian with respect to only K^0, $\bar{K}^0$ states, starting
from the hermitian Hamiltonian. For brevity, we take only a \pi\pi state into
account as the decay channel in this paper. We can not avoid an oscillation
between K^0, $\bar{K}^0$ and \pi\pi states if we start from a hermitian
Hamiltonian whose states all have discrete energy levels. We therefore treat
the \pi\pi state more appropriately to have a continuous energy spectrum to
achieve the decay of K^0, $\bar{K}^0$ into \pi\pi. As the consequence, we find
a different time evolution from what we expect in the conventional method
immediately after the decay starts, though it recovers Fermi's golden rule for
long enough time scale.
