Abstract: A transformer bobbin assembly comprising in combination an outer frame bobbin having outer frame flanges integrally secured thereto at both axial ends of an inner drum thereof, on which a primary coil is arranged, an inner frame bobbin having inner frame flanges integrally secured thereto at both axial ends of an outer drum thereof, on which a secondary coil is arranged, said outer drum of said inner frame bobbin being located in coaxial alignment with the inner drum of the outer frame bobbin so that the inner frame bobbin is mounted over the outer frame bobbin so as to surround the same. Protection covers are secured to the outer frame bobbin to cover the primary and secondary coils as well as the connected portions of the lead wires thereof to the outside circuit.