  In this paper, we study phase structure of a system of hard-core bosons with
a nearest-neighbor (NN) repulsive interaction in a stacked triangular lattice.
Hamiltonian of the system contains two parameters one of which is the hopping
amplitude $t$ between NN sites and the other is the NN repulsion $V$. We
investigate the system by means of the Monte-Carlo simulations and clarify the
low and high-temperature phase diagrams. There exist solid states with density
of boson $\rho={1 \over 3}$ and ${2\over 3}$, superfluid, supersolid and
phase-separated state. The result is compared with the phase diagram of the
two-dimensional system in a triangular lattice at vanishing temperature.
