PURPOSE:To prevents the spring back of laminated iron cores and to produce a cast rotor having wide ventilation ducts, by casting and forming conductors in common use as spacers which are made long in the radial direction of the iron cores between every other laminated iron cores. CONSTITUTION:Laminated iron cores 12 are laminated by each block after cores 13 for forming the spaces for ventilation ducts having conductor forming holes 13a, 13b in common use as spacers are inserted therebetween. The laminated iron cores are put into a die 14, and molten aluminum is cast into the spaces 15 for forming vanes, spaces 16 for forming short-circuiting rings, the slots 12a of the cores 12 and the holes 13a, 13b which are mated so as to communicate with the slots at the top and bottom. The die 14 and the cores 13 are removed upon ending of the casting, whereby the cast rotor is obtd. Every forming holes 13b in the above are formed longer in a radial direction than the holes 13a to form the longer spacers by casting thereby preventing the spring back of the iron cores 12 after the removal of the cores 13 and forming the ventilation ducts having a uniform spacing.