  We report the single-crystal X-ray analysis of the structure of the
pyrochlore oxide superconductor KOs$_2$O$_6$. The structure was identified as
the $\beta$-pyrochlore structure with space group $Fd\bar{3}m$ and lattice
constant $a$ = 10.089(2)~\AA at 300 K: the K atom is located at the 8$b$ site,
not at the 16$d$ site as in conventional pyrochlore oxides. We found an
anomalously large atomic displacement parameter $U_\mathrm{iso}$ =
0.0735(8)~\AA$^2$ at 300 K for the K cation, which suggests that the K cation
weakly bound to an oversized Os$_{12}$O$_{18}$ cage exhibits intensive
rattling, as recently observed for clathrate compounds. The rattling of A
cations is a common feature in the series of $\beta$-pyrochlore oxide
superconductors AOs$_2$O$_6$ (A = Cs, Rb and K), and is greatest for the
smallest K cation.
