Polycrystalline samples of NiGa2S4+delta (-0.2 <delta <+0.2) have been synthesized by solid-state reaction. X-ray powder-diffraction measurements indicate that these samples are single phase with a trigonal lattice with a nominal concentration delta at least between -0.12 and +0.16. Rietveld analyses for time-of-flight neutron-diffraction data have revealed that there is no site mixing between Ni and Ga atoms, indicating that the triangular lattice is only formed by Ni atoms. Our susceptibility measurements have clarified that the freezing temperature and the magnitude of hysteresis systematically change with nominal sulfur concentration and form a respective peak and minimum at the nominal NiGa2S4.04. These results together with our chemical analyses indicate that the actual content of sulfur for the nominal NiGa2S4.04 should be stoichiometric, namely, 4.00. On the contrary, the Ga nuclear quadrupole resonance (NQR) has suggested the presence of two different Ga sites from the Ga-NQR spectra in both samples with the nominal concentration of NiGa2S4.00 and NiGa2S4.04, despite the fact that there is a single crystallographic site for Ga in the unit cell. High-resolution transmission electron microscopy scans conducted over a wide region of a NiGa2S4.04 single crystal found a clean defect-free in-plane structure. Diffraction patterns of the selected area indicate the partial presence of a superlatticelike stacking fault. This suggests that the two different Ga sites are not generated by the sulfur deficiency but by the superlatticelike stacking fault.