  We have searched for $\Theta^{+}$ via $\pi^{-}p \to K^{-}X$ reaction using
1.87 and 1.92 GeV/c $\pi^{-}$ beam at the K2 beam line of the KEK 12 GeV Proton
Synchrotron. In the missing mass spectrum at beam momentum of 1.92 GeV/$c$, a
bump has been found at 1530 MeV/$c^{2}$ which is consistent with the mass
reported by several experiments. The statistical significance of this bump,
however, is only 2.5$-2.7\sigma$. Therefore we have derived the upper limit of
$\Theta^{+}$ production cross section via $\pi^{-}p \to K^{-}\Theta^{+}$
reaction which is 4.3$\mu$b at 90% confidence level assuming that $\Theta^{+}$
is produced isotropically in the center of mass system.
