PURPOSE: To simplify the mounting structure of a core, to reduce the manufacturing cost and to make the whole apparatus compact by integrally molding the core along the inner peripheral wall of a lower drum into the shape of a cylinder and dispensing with a stator holder.

CONSTITUTION: A core 24 is integrally formed into the shape of a cylinder along with a projecting part 24a. The projecting part 24a is mounted on a part 23a of a lower drum 23 where the diameter is increased. The core 24 is fixed inside the lower drum 23 by screwing the part 23a. Therefore, when the core 24 is to be set in the lower drum 23, stator holder 5 a conventionally required, is unnecessary, whereby the core 24 is mounted easily and simply and the number of mounting procedures is reduced. Moreover, since the core 24 is integrally formed with the projecting part 24a, it becomes unnecessary to assemble a plurality of cores 24. Accordingly, because of the reduction of the number of parts and the number of assembling procedures, the manufacturing cost can be decreased and the whole of the rotary transformer device can be made compact in size.