We have calculated the Delta particle-nucleon hole core polarization d
iagram G(pp Delta h) to investigate the effect of the Delta three-nucl
eon force on the shell-model effective interactions. A realistic NN <-
> N Delta transition G-matrix G(N Delta) is derived using a Delta-subt
racted Paris potential and the coupled-channel approach of Lee and Mat
suyama. The core-polarization diagrams G(pp Delta h), second order in
G(N Delta), are found to be very small for the sd-shell valence nucleo
ns. Satisfactory convergence is reached by including Delta excitations
up to 20 oscillator shells for the summation of the Delta intermediat
e states in G(pp Delta h).