  We show that ten(one) years operation of the ultimate DECIGO (DECihertz
Interferometer Gravitational wave Observatory) can determine the cosmic
equation of the state with such accuracy that 0.06%(3%), 0.08%(4%) and
0.06%(3%) for $\Omega_m$, $\Omega_w$ and $w$, respectively. In more realistic
case of practical DECIGO or BBO (Big Bang Observer), $w$ will be determined
within $\sim 10%$ by ten years observation assuming the flat universe model.
Hence, the DECIGO or BBO will give an independent determination of the cosmic
equation of the state.
