This paper reports on the X-ray emission evolution of the ultra-luminous Galactic X-ray pulsar, Swift J0243.6+6124, during the 2017-2018 giant outburst observed by the MAXI GSC. The 2-30 keV light curve and the energy spectra confirm that the luminosity $L_mathrm{X}$ reached $2.5times 10^{39}$ erg s$^{-1}$, 10 times higher than the Eddington limit. When the source was luminous with $L_mathrm{X}gtrsim 0.9times 10^{38}$ erg s$^{-1}$, it exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color ($=$ 4-10 keV / 2-4 keV) and a hard color ($=$ 10-20 keV / 4-10 keV), showed somewhat different behavior across a characteristic luminosity of $L_mathrm{c}simeq 5times 10^{38}$ erg s$^{-1}$. The soft color changed more than the hard color when $L_mathrm{X} < L_mathrm{c}$, whereas the opposite was observed above $L_mathrm{c}$. The spectral change above $L_mathrm{c}$ was represented by a broad enhanced feature at $sim 6$ keV. The pulse profiles made a transition from a single-peak to a double-peak one as the source brightened across $L_mathrm{c}$. These spectral and pulse-shape properties can be interpreted by a scenario that the accretion columns on the neutron star surface, producing the Comptonized X-ray emission, gradually became taller as $L_mathrm{X}$ increased. The broad 6 keV enhancement could be a result of cyclotron-resonance absorption at $sim 10$ keV, corresponding to a surface magnetic field $B_mathrm{s}simeq 1.1times 10^{12}$ G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth correlation with $L_mathrm{X}$ up to the outburst peak, and its linear coefficient is comparable to those of X-ray binary pulsars whose $B_mathrm{s}$ are $(1-8)times 10^{12}$ G. These results suggest that $B_mathrm{s}$ of Swift J0243.6$+$6124 is a few times $10^{12}$ G.
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