PURPOSE: To provide a coil device of small size which can be manufactured easily by a method wherein the coil device consists of a primary winding and a secondary winding which are wound around a ferrite core directly or through a bobbin and the whole or a part of exposed surface of the ferrite core is covered with an insulating material.

CONSTITUTION: Two E-shaped ferrite cores comprising an insulating coat 8 respectively are attached to a resin made bobbin 2 as a pair of upper and lower cores. Then, a primary winding 3 and a secondary winding 4 are wound between a pair of flanges 2a and 2b of the bobbin 2. Over the whole surface of ferrite core 1, an insulating coat 8 made of a synthetic resin such as epoxy, polyimide, silicon, Teflon or the like is formed by spreading or immersion. As the withstand voltage of the transformer is increased by thus forming the insulating coat 8 made of synthetic resin or synthetic rubber over the whole surface of ferrite core 1, it is not necessary to regard a distance between the primary winding 3 (secondary winding 4) and the ferrite core 1.