Abnormal flows such as an entrainment or an overflow have been restricted severely in the design and operation of the solvent extraction process. The effect of the abnormal flows on the process has been studied for a PUREX flow sheet to find a way to manage their unfavorable effects on the process. As one of these activities, the algebraic equation expressing the concentration of the outflow by the operational conditions for a countercurrent solvent extraction system with the existence of abnormal flows was derived. This equation was the expanded form of the conventional Kremser's equation, which has been applied to the flow sheet under normal conditions. The preliminary study using the equation is also shown.