  We report a novel phenomenon intimately related to the spin-triplet
superconductivity. It is well known that the spin susceptibility decreases
below the superconducting transition temperature in almost all superconductors
because of spin-singlet pair formation, while it may remain unchanged in a
handful of spin-triplet exceptions. Here we report the observation in
Sr$_2$RuO$_4$ with nuclear magnetic resonance (NMR) that the spin
susceptibility originating from the Ru-4$d$ electron slightly $increases$ by
$\sim 2 $\% of total and becomes inhomogeneous in the superconducting state.
These are reasonably explained if the electron pairs form the
equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted
for superfluid $^3$He forty years ago, but had never been demonstrated in any
superconductor.
