PURPOSE: To minimize the reduction of the volume of a superconducting field coil and permit the increase of an insulating distance efficiently, by providing insulating materials having L-shape sections at four corners between the inner surface of a slot and the outer surface of the superconducting field coil so as to surround the superconducting field coil.

CONSTITUTION: A rotor is constituted of a superconducting field coil 3, received in a slot 16 formed on a coil mounting shaft 2 through an insulating material 22 in the slot. Sheet type insulating materials 21, having L-shape sections, are arranged at the four corners of the insulating material 22 in the slot so as to surround the field coil 3. Upper and lower pads 22a, 22b and a side pad 22c, which constitute the insulating material 22 in the slot, are provided with notches 25 while the L-shape insulating materials 21 are received in the notches 25. In such constitution of insulating materials, an insulating distance La between the field coil 3 and a wedge 15 may be shown by a formula in a diagram whereby the insulating distance between the wedge 15 may be increased by (la-Db-t).