PURPOSE: To attempt to save expensive condensable cooling liquid by filling up a space between a winding and tank and a space between an iron core and the tank with forming resin.

CONSTITUTION: A container 14 is composed by providing a cylinder unit 14a made of insulation material around the outer circumference of a primary winding 2a with a duct 13 interposed between them and installing a bottom unit made of insulation material, which is arranged under a winding 2 with a duct 15 interposed between them, to the bottom of the cylinder unit 14a. Vacant parts to be formed by the container 14 and an iron core 1, the coil 2 and a tank 5 are filled up with forming resin 16. In other words, cooling liquid 11 is caused to fill up ducts 3, 4, 13, 15 and a part above the coil 2 which are required to cool the heat generating elements of the coil 2 and the iron core 1. By this, the cost of a transformer can be reduced by saving cooling liquid 11.