Se-77 NMR signals were observed in the metallic and insulating states of the quasi-one-dimensional organic compound (DIETSe)(2)GaCl4. The Se-77 NMR spectra at low temperatures are broadened, indicating the presence of magnetic ordering. The shape of the spectra in the magnetically ordered state is characteristic of the spin density wave (SDW) ordering. The divergent behavior of the temperature dependence of the nuclear spin-lattice relaxation rate suggests that the SDW ordering occurs at about 7 K which is lower than the metal-insulator transition temperature of 12 K.