  Using the SU6 quark-model baryon-baryon interaction recently developed by the
Kyoto-Niigata group, we calculate NN, Lambda N and Sigma N G-matrices in
ordinary nuclear matter. This is the first attempt to discuss the Lambda and
Sigma single-particle potentials in nuclear medium, based on the realistic
quark-model potential. The Lambda potential has the depth of more than 40 MeV,
which is more attractive than the value expected from the experimental data of
Lambda-hypernuclei. The Sigma potential turns out to be repulsive, the origin
of which is traced back to the strong Pauli repulsion in the Sigma N (I=3/2)
^3S_1 state.
