Electric field dependence of the output polarization was investigated in the dye laser light amplifier, using the dimethyl sulfoxide (DMSO) solution of coumarin 30, which was found to have a large Kerr coefficient and a high resistivity. Output polarization was found to be determined by the Kerr effect of the solvent DMSO and the anisotropic gain due to the anisotropic orientation distribution of excited dye molecules. The nonlinear electric susceptibility ‖&khgr;xxxx−&khgr;zzxx‖ of DMSO was estimated to be 9.5×10−12cgs esu. The orientational distribution time constant &tgr;rfor coumarin 30 dissolved in DMSO was estimated to be about 1.3×10−9s.