  The exotic baryon $\Theta^+(uudd\bar{s})$ is studied with microscopic
calculations in a quark model by using a method of antisymmetrized molecular
dynamics. We predict that the lowest even state is $J^\pi=1/2^+(I=0)$ or
$J^\pi=3/2^+(I=0)$, and the lowest odd state is $J^\pi=3/2^-(I=1)$. They nearly
degenerate in the $uudd\bar{s}$ system. We discuss $K^+n$ decay widths and
estimate them to be $\Gamma< 7$ for the $J^\pi=\{1/2^+,3/2^+\}$, and $\Gamma<1$
MeV for the $J^\pi=3/2^-$ state.
