We report the first high-pressure 11B-NMR studies above 3 GPa on the intermediate-valence semiconductor SmB6. A 11B-NMR line obtained at 4.9 GPa, the highest pressure for the measurements, and at 1.9 K shows quite similar a line shape to that at ambient pressure, indicating no structural or magnetic phase transition up to this pressure. The temperature dependence of the spin lattice relaxation rate 1/T 1 at 4.9 GPa still exhibits an activation-type temperature dependence characteristic of semiconductors, which reveals an obvious decrease in the insulating gap by about 30% compared to the gap at ambient pressure. The present experimental facts of a finite insulator gap and no magnetic order at 4.9 GPa are consistent with recent transport measurements performed under better hydrostatic pressures.