In order to obtain a superior thixotropic property even in a high strength 7075 aluminum alloy a billet of an Al-5.5%Zn-2.5%Mg-1.5%Cu alloy with a nondendritic fine-grained structure has been tried to produce by combining a simple mechanical stirring with the vertical semi-continuous casting process. We succeeded to get a good quality billet on the basis of the trials that had been carried out under the various stirring conditions, especially focussing on the position of a stirring rotor, the shape of a rotor and the volume of a stirring chamber. When the temperature within a stirring chamber was nearly constant just below the liquidus temperature and dropped drastically near the liquid-solid interface during casting, a long sound billet with relatively smooth surface could be produced. And the billet had not only a uniformly fine-grained structure with the average grain size of similar to 30 mu m, but also little macroscopic segregation of the solute elements along the radius at the various positions of the billet.