The equations of atomic density matrix are solved without any approximation when the strong:ignal field and external coherence-driving field are detuned to respective dipole-allowed transition of the Λ atom in such a way that Δ1=-Δ2. Under the equal Rabi frequency (G) condition, it is found that approximate, coherent population trapping appears for small detuning, which leads to the transparency of the medium to the strong signal having an order of Ｏ（Ｇ-3）. If the detuning is G/2, the atom nearly equally populates three levels and the transparency is of only order Ｏ（Ｇ-1）.