Abstract: An apparatus for assembling a fuel rod bundle having a number or elongated fuel rods arranged in parallel with each other and fixed by means of a plurality of spacers and lower and upper tie plates comprises a fuel rod supplying apparatus for feeding successive fuel rods in a longitudinal direction of the rods, said fuel rod supplying apparatus including means for carrying the rods in a direction perpendicular to the longitudinal direction to any one of a plurality of positions; a bundling apparatus including means for fixing or holding the spacers and the lower tie plate and means for rotating said fixing means by given angles; and a measuring apparatus including a circular pitch measuring mechanism and a layer space or pitch measuring mechanism for measuring circular and layer pitches of the assembled fuel rods in the bundling apparatus.