We report high-sensitivity microwave measurements of the in-plane penetration depth $lambda_{ab}$ and quasiparticle scattering rate $1/tau$ in several single crystals of hole-doped Fe-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($xapprox 0.55$). While power-law temperature dependence of $lambda_{ab}$ with the power $sim 2$ is found in crystals with large $1/tau$, we observe exponential temperature dependence of superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different level of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe-arsenides.
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