The joining device (100, 100′) comprises a first cover (1) and a second cover (2), each of which has a plurality of through holes (3) that match up with one another in position, for the ends of the rebars of the columns (200, 200A) to pass through. The covers (1,2) are joined together by means of, at least, two transverse webs (4) and several stiffener reinforcements (5,5A), which are arranged in perpendicular to the covers (1,2). The device further comprises connecting means (202, 203, 204, 205) between the ends of the rebars (201,201A) of the columns (200,200A).