  We study the spin-orbit splitting ($ls$-splitting) for the proton d-orbits in
23F in the Hartree-Fock framework with the tensor force in the exchange
channel. 23F has one more proton around the neutron-rich nucleus 22O. A recent
experiment indicates that the ls-splitting for the proton d-orbits in 23F is
reduced from that in 17F. Our calculation shows that the ls-splitting in 23F
becomes smaller by about a few MeV due to the tensor force. This effect comes
from the interaction between the valence proton and the occupied neutrons in
the 0d5/2 orbit through the tensor force and makes the ls-splitting in 23F
close to the experimental data.
