PURPOSE: To prevent trouble caused by dielectric breakdown or the obstruction of a cooling duct in a transformer by a method wherein an air bubble is detect- ed by providing a floating flow meter in a liquid collecting pipe or a cooling duct for the flow of a cooling medium installed within the coil of the transformer.

CONSTITUTION: A cooling medium 7 pressurized and circulated by a pump 8 is made to enter a cooling duct 6 after passing through a lower liquid collecting pipe 11, increasing it temperature because of the heat radiated from a foil wound copper conductor 2, then passing through a floating flow meter 15 after the liquid collecting pipe 11 and to return to the pump 8 cooled by a cooler 9. When a fluid containing a mixture of air bubbles and the liquid passes through the flating flow meter 15, the air bubbles act to decrease the area where the fluid passes, causing violent vibrations and thus having the fact detected that air bubbles have passed therethrough. In this way, the trouble caused by dielectric breakdown or the obstruction of the cooling duct in the transformer can be prevented by detecting the air bubbles generated in the flow passage of the cooling medium.