  In-plane microwave penetration depth $\lambda_{ab}$ and quaiparticle
conductivity at 28 GHz are measured in underdoped single crystals of the
Fe-based superconductor PrFeAsO$_{1-y}$ ($T_c\approx 35$ K) by using a
sensitive superconducting cavity resonator. $\lambda_{ab}(T)$ shows flat
dependence at low temperatures, which is incompatible with the presence of
nodes in the superconducting gap $\Delta({\bf k})$. The temperature dependence
of the superfluid density demonstrates that the gap is non-zero
($\Delta/k_BT_c\gtrsim 1.6$) all over the Fermi surface. The microwave
conductivity below $T_c$ exhibits an enhancement larger than the coherence
peak, reminiscent of high-$T_c$ cuprate superconductors.
