  A rare decay $K_L \to \mu^+ \mu^- $ has been measured precisely, while a rare
decay $K_S \to \mu^+ \mu^- $ will be observed by an upgrade of the LHCb
experiment. Although both processes are almost CP-conserving decays, we point
out that an interference contribution between $K_L$ and $K_S$ in the kaon beam
emerges from a genuine direct CP violation. It is found that the interference
contribution can change $K_S \to \mu^+ \mu^-$ standard-model predictions at
$\mathcal{O}(60\%)$. We also stress that an unknown sign of $\mathcal{A}(K_L
\to \gamma \gamma)$ can be determined by a measurement of the interference,
which can much reduce a theoretical uncertainty of $\mathcal{B}(K_L \to \mu^+
\mu^-)$. We also investigate the interference in a new physics model, where the
$\epsilon'_K / \epsilon_K$ tension is explained by an additional $Z$-penguin
contribution.
