A solvent extraction process experimental apparatus was designed as an engineering scale test facility for the TRU extraction process of four-group partitioning process. This apparatus has a feature that a variable bank-stage mixer-settler fixed together 2 or 4 stages banks makes it possible to examine the solvent extraction characteristics with various sets of organic and aqueous solutions. A demonstration test used a simulated solution with La and Nd showed that the variable bank-stage mixer-settler had an expected extractability of TRU from the solution, irrespective of the number of bank-stage. In addition, the leakage and discontinuity of flow patterns of organic and aqueous solutions were not found at the joint of the banks in the mixer-settler. We may conclude that this apparatus would be useful enough to examine the extraction characteristics of TRU by various organic solvents on an engineering scale. (author).