  We carefully calculate the nucleon-nucleon interaction due to two pion
exchange processes by properly evaluating the corresponding Feynman diagrams.
In the estimation, we have made no approximation, and instead, we carry out the
numerical integrations of three Feynman parameters. It is found that the two
pion exchange potential gives rise to the attractive force which corresponds to
the effective scalar meson with its mass of $m_s\simeq 4.7 m_\pi$ and its
strength of ${g_s^2\over 4\pi}\simeq 1.45 $. There is a strong isospin
dependence of $(\bm{\tau}_1\cdot \bm{\tau}_2)^2 $ which cannot be simulated by
the one boson exchange model calculations.
