We measured the electrical resistivity of black SmS at high magnetic fields up to 30 T. We observed that the magnetic field drastically reduces the energy gap E_g (H, P) in a similar way to that the pressure does. From the finding that data points of E_g(H, P) lie on a single line as functions of H and P, we argue that the giant negative magnetoresistance is intrinsic to black SmS and the magnetic field plays a similar role to the pressure.