To enable high-efficiency cooling of wires, without reducing the size of a spacer by forming unsymmetrical grooves on both sides of the spacer, in the winding structure of a transformer to establish path channels for coolant in contact planes of the wires.

A groove in the center along longitudinal direction and two grooves in short side direction of a spacer 1 are formed to prepare a spacer oil path 2. A spacer oil path 2 is also formed on the backside of the spacer 1, wherein the shorter parts of the spacer oil path are made unsymmetrical so that their position are not overlapped between front-side and backside. If the spacer oil paths are made tilted, fear of dropping down of wires into the grooves is eliminated even if the wires are thin, and wires are made difficult to be damaged, even if mechanical force is applied. Coolant can pass through spaces among several tens of the spacers 1 positioned radiating in the entire periphery at regular angular intervals and through the oil paths 2. As a consequence, parts of the wires between which the spacer 1 is inserted can dissipate heat to the coolant, which creeps in through the spacer oil paths 2. Also, oil temperature can be homogenized by the flow of the coolant through the spacer oil paths 2 along the circumferential direction of the wires.