A high-pressure powder x-ray-diffraction experiment has been carried out on Ga up to 150 GPa at room temperature. The $c/a$ axial ratio of the body-centered-tetragonal phase III continuously decreases with pressure and becomes $\sqrt{2}$ at $120\ifmmode\pm\else\textpm\fi{}10\mathrm{GPa},$ where an fcc lattice is realized. There is no detectable volume change at the phase transition. The fcc phase Ga(IV) is stable to at least 150 GPa. Full-potential linearized augmented plane-wave calculations have been done to investigate the pressure dependence of the $c/a$ axial ratio of phase III. The result of the calculation is in reasonable agreement with the present experiment. The energy band structure of fcc Ga has also been calculated with the pseudopotential method. The valence bands are found to touch the $3d$ core states at about 79 GPa.