  The electromagnetic contribution to the $K^+ - K^0$ mass difference is
calculated in the $1/N_c$ approach including the $SU(3)$ breaking contributions
due to a non-zero strange quark mass. The short-distance contribution can be
unambiguously determined in terms of the known parameters of the
next-to-leading order chiral lagrangian. The long distance part is estimated
using the $1/N_c$ approach by Bardeen et al. Relatively large corrections are
found.
