  We report on a frequency ratio measurement of a ${}^{199}$Hg-based optical
lattice clock referencing a ${}^{87}$Sr-based clock. Evaluations of lattice
light shift, including atomic-motion-dependent shift, enable us to achieve a
total systematic uncertainty of $7.2 \times 10^{-17}$ for the Hg clock. The
frequency ratio is measured to be $\nu_{\rm Hg}/\nu_{\rm Sr}=2.629\, 314\,
209\, 898\, 909\, 60(22)$ with a fractional uncertainty of $8.4 \times
10^{-17}$, which is smaller than the uncertainty of the realization of the SI
second, i.e., the SI limit.
