PURPOSE: To improve purity, by adding an alcohol, distilling and separating the titled compound in purifying a trialkylaluminum-containing trialkylindium to provide the titled compound which is a raw material used for synthesizing a compound semiconductor of indium with a group Va element.

CONSTITUTION: An alcohol is added to a trialkylindium containing a trialkylaluminum and reacted at a temperature within the range of 0W150°C under ordinary pressure, as necessary, under a higher or reduced pressure for 10minW2hr. After completing the reaction, distillation and separation are carried out to afford the aimed compound. The distillation is preferably vacuum distillation. Methyl alcohol, isobutyl alcohol, etc., are cited as the above-mentioned alcohol used and the amount thereof added is preferably within the range of 1.5mol. based on 1mol. mixed trialkylaluminum. Furthermore, the afore- mentioned raw material is obtained by subjecting an indium halide with the trialkylaluminum to exchange reaction, followed by distillation and separation.