Stage efficiency of a mixer-settler extraction column which could realize a larger throughput at a higher agitation speed was measured with a three-stage column for the extraction of iodine from aqueous phase into heptane. The efficiency increased monotonously with agitation speed; i.e., high-stage efficiency could be obtained as well as a large throughput at a high agitation speed with the extraction column. The stage efficiency based on the dispersed-phase concentration varied with the ratio of slopes between the equilibrium line and the operating line. The overall height of a transfer unit transformed from the stage efficiency was divided into the heights of a transfer unit of the dispersed and the continuous phases, which were correlated with the agitation speed.