  Using an unbiased quantum Monte Carlo method, we obtain convincing evidence
of the existence of a checkerboard supersolid at a {\it commensurate} filling
factor 1/2 (commensurate supersolid) in the soft-core Bose-Hubbard model with
nearest-neighbor repulsions on a cubic lattice. In conventional cases,
supersolids are realized at incommensurate filling factors by a
doped-defect-condensation mechanism, where particles (holes) doped into a
perfect crystal act as interstitials (vacancies) and delocalize in the crystal
order. However, in the above model, a supersolid state is stabilized even at
the commensurate filling factor 1/2 {\it without doping}. By performing grand
canonical simulations, we obtain a ground-state phase diagram that suggests the
existence of a supersolid at a commensurate filling. To obtain direct evidence
of the commensurate supersolid, we next perform simulations in canonical
ensembles at a particle density $\rho=1/2$ and exclude the possibility of phase
separation. From the obtained snapshots, we discuss the microscopic structure
and observe that interstitial-vacancy pairs are unbound in the crystal order.
