PURPOSE: To provide a method of preventing moisture absorption of the windings of a transformer, which is simple in constitution, on which a sealing work is easily conducted and which is economical.

CONSTITUTION: The whole windings 1 of a transformer or the outer peripheral surface and upper and lower surfaces of the interior of a transformer, in which windings 1 are respectively wound on each core, are covered with oil-resistant insulating films 2 and 3, the bonded parts of the films 2 and 3 are made to lap each other to fix and seal the bonded parts by bands 4 and 5 and moisture absorption of the windings of the transformer with its interior, which is covered with the films 2 and 3 and in which dried air is fed, is prevented.