  We have prepared polycrystalline samples of (Sr$_{1-x}$La$_x$)$_2$ErRuO$_6$
and (Sr$_{1-x}$La$_x$)$_2$YRuO$_6$, and have measured the resistivity, Seebeck
coefficient, thermal conductivity, susceptibility and x-ray absorption in order
to evaluate the electronic states and thermoelectric properties of the doped
double-perovskite ruthenates. We have observed a large Seebeck coefficient of
-160 $\mu$V/K and a low thermal conductivity of 7 mW/cmK for $x$=0.1 at 800 K
in air. These two values are suitable for efficient oxide thermoelectrics,
although the resistivity is still as high as 1 $\Omega$cm. From the
susceptibility and x-ray absorption measurements, we find that the doped
electrons exist as Ru$^{4+}$ in the low spin state. On the basis of the
measured results, the electronic states and the conduction mechanism are
discussed.
