We report on a frequency ratio measurement of a Hg199-based optical lattice clock referencing a Sr87-based clock. Evaluations of lattice light shift, including atomic-motion-dependent shift, enable us to achieve a total systematic uncertainty of 7.2×10−17 for the Hg clock. The frequency ratio is measured to be νHg/νSr=2.629 314 209 898 909 60(22) with a fractional uncertainty of 8.4×10−17, which is smaller than the uncertainty of the realization of the International System of Units (SI) second, i.e., the SI limit.
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