The dependence of the magneto-optical readout signal on readout power and external field during the readout operation was examined in order to obtain the critical readout power (the upper limit below which no change occurs in the written domains) and the magnetization reversal process above this critical power. The critical readout power became maximum when an external field was applied to suppress the force acting on the wall of the written domains. The domain length along the recording track changed when the readout operation was above the critical readout power. The domain expanded with increasing readout power and the domain finally changed to a reversed stripe when an external field was applied to enhance the expansive force on the wall, while the domain shrunk and disappeared when a field was applied to enhance the contractive force. It has been shown that external field application during the readout operation assisted in obtaining a wide readout power margin. It has also been shown that magnetization reversal during the readout operation obeyed the domain wall motion process rather than the nucleation process.