Measuring the Leptonic CP Phase in $\nu_{\mu} \to \nu_{\mu}$ Oscillations

In $\nu_{\mu} \to \nu_{\mu}$ oscillations, we find that the effect of the CP phase $\delta$ becomes large in the region $E<2$ GeV and $L>2000$ km. In this region, the change of the probability in this channel reaches about 0.4 due to the CP phase effect beyond our expectation in the case of large 1-3 mixing angle. Furthermore, the CP phase effect have almost same sign over the region $E>0.5$ GeV so that one may find the signal of CP violation by measuring the total rate only. As an example, we use an experimental setup and demonstrate that the allowed region is limited to one by combined analysis of $\nu_e$ and $\nu_{\mu}$ events although there remain three allowed regions by the analysis of $\nu_e$ events alone.