PURPOSE: To make it possible to easily save expensive condensable cooling liquid by such an arrangement wherein a space to be formed by a container covering the outer circumference and underside of a winding and a tank is filled up with thermosetting resin and the resin is caused to be hardened when the interior of the tank is made vacuous, heated and dried.

CONSTITUTION: Internal components of a transformer such as an iron core 1, a winding 2, a container 14, etc. are assembled, and under this condition, the gap between the iron core 1 and the bottom 14b and the gap between a cylinder unit 14a and the bottom 14b are sealed up with a sealing agent 17. Next, the assembly is placed in a tank 5 and a specified amount of thermosetting type resin 16 which is in liquid form under normal temperature is poured into a space formed by the tank 5 and the container 14 and then the resin 16 is caused to be hardened by making the tank vacuous and heating it. Normally, a transformer is made vacuous, heated and dried to remove moisture from the iron core 1, the coil 2 and interior of the tank 5 and improve its voltage-withstand characteristic, and the heating of the resin 16 for causing it to harden can be carried out simultaneously with the vacuous heating and drying of the transformer. By this method, the space can be easily filled up.