  The mass and decay width of the $\Theta^+(1540)$ with isospin 0 are
investigated in a constituent quark model comprising $uudd\bar{s}$ quarks. The
resonance state for the $\Theta^+$ is identified as a stable solution in
correlated basis calculations. With the use of a one-gluon exchange quark-quark
interaction, the mass is calculated to be larger than 2 GeV, increasing in
order of the spin-parity, ${1/2}^-, {3/2}^-$ and ${1/2}^+ ({3/2}^+)$, and only
the ${3/2}^-$ state has a small width to the $nK^{*+}$ decay. If the calculated
mass is shifted to around 100 MeV above the $N+K$ threshold, the
$\Theta^+(1540)$ is possibly ${1/2}^+ ({3/2}^+)$ or ${3/2}^-$, though in the
latter case it cannot decay to the $nK^{+}$ channel. In addition it is
conjectured that other pentaquark state with different spin-parity exists below
the $\Theta^+(1540)$. The structure of the $\Theta^+$ is discussed through the
densities and two-particle correlation functions of the quarks and through the
wave function decomposition to a baryon-meson model and a diquark-pair model.
