We have observed the prompt emission of GRB100418A, from its beginning by the MAXI/SSC (0.7-7 keV) on board the International Space Station followed by the Swift/XRT (0.3-10 keV) observation. The light curve can be fitted by a combination of a power law component and an exponential component (decay constant is $31.6pm 1.6$ sec). The X-ray spectrum is well expressed by the Band function with $E_{rm p}leq$8.3 keV. This is the brightest GRB showing a very low value of $E_{rm p}$. It satisfies the Yonetoku-relation ($E_{rm p}$-$L_{rm p}$). It is also consistent with the Amati relation ($E_{rm p}$-$E_{rm iso}$) in 2.5$sigma$ level.
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