  Polar Kerr effect in the spin-triplet superconductor Sr2RuO4 was measured
with high precision using a Sagnac interferometer with a zero-area Sagnac loop.
We observed non-zero Kerr rotations as big as 65 nanorad appearing below Tc in
large domains. Our results imply a broken time reversal symmetry state in the
superconducting state of Sr2RuO4, similar to He3-A.
