To provide a device where insulating gas does not flow reversely.

In a natural cooling system of gas insulated transformer composed of a sealed container 1 charged with insulating gas 10 and accommodating the main body 2 of a transformer, a heat radiator 3 installed separately from this sealed container 1, an upper pipe 4 for connecting the sealed container 1 with the top of the heat radiator 3, and a lower pipe 5 for connecting the sealed container 1 with the bottom of the heat radiator 3, a check valve 11 is interposed in the lower pipe 5, and only the insulating gas 10 flowing to the main body 2 of a transformer 2 through the lower pipe 5 from heat radiator 5 is let pass this check valve 11. This way, the reverse flow of insulating gas can be suppressed by the check valve 11 having being interposed in the lower pipe or the upper pipe, and trouble ceases to occur such as that the transformer fails to operate due to the abnormal temperature rise of the main body 2 of the transformer. Moreover, in such constitution, the check valve can be made simply and besides at a low price of a net and a film, and it can open and close without fail.