Most of aluminum circular tubes were formed by extrusion processes. The circular tube has high flexural rigidity and torsional rigidity as compared with its weight. Therefore, tube consumption is increasing for reducing industrial parts' weight in many products. The shape of tube cross section must be changed and manufactured in proportion to the magnitude of moment, which will act on them. Therefore, we have invented and made a new prototype extrusion machine which can control the inside diameter of circular tubes, and this study is the one of the extrusions with changing cross section shape. The possibility and characteristics of the extrusion using tapered mandrel were experimentally made clear in this study.