The solvent extraction of alkali and alkaline earth metals (Li, Na, K, Mg, Ca) was studied using organophosphorus compounds like di-2-ethylhexylphosphoric acid(D2EHPA) or 2-ethylhexyl-phosphonic acid 2-ethylhexyl ester (MEHPA) as extractants. Extraction equilibrium constants and composition of each metal complex in organic phase were determined. Synergistic effect of tri-n-butylphosphate (TBP) was observed for lithium and sodium. This effect was presumed to be due to the replacement of solvated extractant(D2EHPA or MEHPA) by TBP. Finally, lithium was recovered by the above solvent extraction system from hot spring water in Oita, Japan.