  We present the temperature dependence of the thermal conductivity kappa(T) of
the unconventional superconductor Sr_2RuO_4 down to low temperatures (100 mK).
In T -> 0 K limit we found a finite residual term in kappa/T, providing clear
evidence for the superconducting state with an unconventional pairing. The
residual term remains unchanged for samples with different T_c, demonstrating
the universal character of heat transport in this spin-triplet superconductor.
The low-temperature behavior of kappa suggests the strong impurity scattering
with a phase shift close to pi/2. A criterion for the observation of
universality is experimentally deduced.
