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This paper reports on the X-ray emission evolution of the ultra-luminousGalactic X-ray pulsar, Swift J0243.6+6124, during the 2017-2018 giant outburstobserved by the MAXI GSC. The 2-30 keV light curve and the energy spectraconfirm that the luminosity $L_\mathrm{X}$ reached $2.5\times 10^{39}$ ergs$^{-1}$, 10 times higher than the Eddington limit. When the source wasluminous with $L_\mathrm{X}\gtrsim 0.9\times 10^{38}$ erg s$^{-1}$, itexhibited a negative correlation on a hardness-intensity diagram. However, twohardness 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 acharacteristic luminosity of $L_\mathrm{c}\simeq 5\times 10^{38}$ 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 $L_\mathrm{c}$. Thespectral change above $L_\mathrm{c}$ was represented by a broad enhancedfeature at $\sim 6$ keV. The pulse profiles made a transition from asingle-peak to a double-peak one as the source brightened across$L_\mathrm{c}$. These spectral and pulse-shape properties can be interpreted bya scenario that the accretion columns on the neutron star surface, producingthe Comptonized X-ray emission, gradually became taller as $L_\mathrm{X}$increased. The broad 6 keV enhancement could be a result of cyclotron-resonanceabsorption at $\sim 10$ keV, corresponding to a surface magnetic field$B_\mathrm{s}\simeq 1.1\times 10^{12}$ G. The spin-frequency derivativescalculated with the Fermi GBM data showed a smooth correlation with$L_\mathrm{X}$ up to the outburst peak, and its linear coefficient iscomparable 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 SwiftJ0243.6$+$6124 is a few times $10^{12}$ G.

X-ray emission evolution of the Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124 during the 2017-2018 outburst observed by the MAXI GSC