in concrete mixing plant a biaxial concrete mixer comprising first and second agitating shafts parallel to each other, a stirring blade and a horizontal mixing tank which extend from the first and second stirring axes and are equidistantly arranged in the axial direction And the agitating blades on the first and second agitating shafts are respectively formed in a spiral-like arrangement in opposite directions in a circumferential direction or counterclockwise at a fixed angle in the circumferential direction; the first and second stirring shafts are rotated synchronously with each other and The blades being staggered through a plane defined by the first and second stirring shaft axes; a feed port is provided at the top of one end of the mixing tank and a discharge port is provided at the bottom of the other end of the mixing tub.