  Magnetic properties in the quasi-one-dimensional organic salt (TMTTF)2SbF6
are investigated by 13C NMR under pressures. Antiferromagnetic phase transition
at ambient pressure (AFI) is confirmed. Charge-ordering is suppressed by
pressure and is not observed under 8 kbar. For 5 < P < 20 kbar, a sharp
spectrum and the rapid decrease of the spin-lattice relaxation rate 1/T1 were
observed below about 4 K, attributed to a spin-gap transition. Above 20 kbar,
extremely broadened spectrum and critical increase of 1/T1 were observed. This
indicates that the system enters into another antiferromagnetic phase (AFII)
under pressure. The slope of the antiferromagnetic phase transition temperature
T_AFII, dT_AFII/dP, is positive, while T_AFI decreases with pressure. The
magnetic moment is weakly incommensurate with the lattice at 30 kbar.
