This paper reports on the X-ray emission evolution of the ultraluminous Galactic X-ray pulsar Swift J0243.6+6124 during the giant outburst from 2017 October to 2018 January as observed by the MAXI GSC all-sky survey. The 2-30keV light curve and the energy spectra confirm the source luminosity LX assuming an isotropic emission reached 2.5x10^39^erg/s, 10 times higher than the Eddington limit for a 1.4M_{sun} neutron star. When the source was luminous with L_X>~0.9x10^38^erg/s, it generally exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color (=4-10keV/2-4keV) and a hard color (=10-20keV/4-10keV), showed somewhat different behavior across a characteristic luminosity of L_c_~5x10^38^erg/s. The soft color changed more than the hard color when L_X_