When a He-Ne laser beam (632.8 nm) with the power of milliwatts or submilliwatts penetrates through a Mn doped KNSBN crystal, a phenomenon of strong backward scattering is observed. At the same time, an optical diode effect which is associated with the backward scatering is also observed. In a certain range, the response time constant T of the optical diode effect is inversely propotional to the incident intensity on the surface of the crystal. The τ is decreased by one order of magnititude when a K9 glass plate which serves as a feedback mirror is introduced. A time constant of 1.5s and a ratio of the forward transmissivty to the backward one, which is greater than 15.3, have been obtained. A theoretical explanation is given on the basis of the experimental results