  The competition between the isovector (T=1) and isoscalar (T=0)
proton-neutron ($p-n$) correlations in $N=Z$ nuclei is investigated by
calculating their correlation energies with a realistic effective interaction
which reproduces observed nuclear properties very well, in a strict shell model
treatment. It is shown in the realistic shell model that the double-differences
of binding energies $(B(A+pn:T) + B(A) - B(A+p) - B(A+n))$ ($B(A)$ being the
binding energy) are good indicators of the T=1 and T=0 $p-n$ correlations. Each
of them, however, originates in plural kinds of correlations with T=1 or T=0.
