PURPOSE: To prevent creeping stress from arising on an electrostatic ring end by using flanges thicker than insulating tubes on inner and outer peripheries of a winding so as to allow DC field to work nearly evenly on all the solid insulators without concentrating on an electrostatic ring insulation of the winding end.

CONSTITUTION: Since the thickness of solid insulations between 100% potential and 0% potential is increased by that of an electrostatic ring insulation 4, a DC field distributed nearly evenly on insulating tubes 5W8 moves to the electrostatic ring insulation 4 at the end due to, on the other hand, a difference in density of a leakage current. However, the field move to the insulation 4 is lightened as the flange thickness has been increased by that of flanges 5a, 5bW8a, 8b. A creeping stress at the end of an electrostatic ring 3 is decreased consequently. The field move between flanges 5a, 5bW8a, 8b increases by the field having been decreased for concentration to the insulation 4, however, the stress creeping on the flange surfaces can be retained at a value reasonably low as the width is large as compared with the thickness of the electrostatic ring 3 or by increasing number of steps of the flanges as occasion demands.