PURPOSE: To provide an economical coaxial type rotary transformer with improved assembly operation without producing any defect such as a marking by using a rotor core with a screw-shaped coil groove structure on the inner periphery surface of the rotor core at the outer periphery side of a coaxial type rotary transformer.

CONSTITUTION: Several turns of screw-shaped coil grooves 2 are cut into a rotor core 1. Two penetration holes 3 are provided at both final grooves of the screw-shaped coil groove 2 facing the outer periphery side of a core, thus penetrating up to a cut-out groove 5 on the outer periphery surface of the core. In a through hole 3, a through hole taper part 4 expanding the output part reaching the cut-out groove on the outer periphery surface of the core in taper shape is provided. A silver paste 6 is coated to this screw-shaped coil groove 2 and the through hole 3 instead of the wire and is thermally cured. The cured silver paste 6 passes through the through hole 3, reaches the surface of the cut-out groove 5, thus forming the coil and a lead terminal of the coil. According to this configuration, the lead part and the terminal part are simultaneously formed when machining the coil, the coil can be assembled fully and can be automated easily.