  We study neutrino oscillation within the framework of three generations in
matter. We propose a simple method to approximate the coefficients A, B and C
which do not depend on the CP phase \delta in the oscillation probability
P(\nu_e \to \nu_{\mu})=A\cos \delta + B\sin \delta +C. An advantage of our
method is that an approximate formula of the coefficients A, B and C in
arbitrary matter {\it without the usual first order perturbative calculations}
of the small parameter \Delta m_{21}^2/\Delta m_{31}^2 or \sin \theta_{13} can
be derived. Furthermore we show that all the approximate formulas for low,
intermediate and high energy regions given by other authors in constant matter
can be easily derived from our formula. It means that our formula is applicable
over a wide energy region.
