  We define a difference $D_{\rm CP}$ of the neutrino oscillation probability
differences with matter effect for the CP-conjugate channels, divided by
neutrino beam energy, taken between the two baselines $L = L_1$ and $L =L_2$
with $L_1/E_1 = L_2/E_2$, where $E_1$ and $E_2$ are the neutrino energy for the
experiment with $L_1$ and $L_2$, respectively. The quantity $D_{\rm CP}$
doesn't contain the matter effect to the first order in $aL/2E$, $a$
representing the matter effect. We show the behavior of $D_{\rm CP}$ with $L_1
= 300$ km fixed and $L_2$ variable in the three-neutrino model.
