A gas tank is hermetically filled with an insulating gas. A gas filling opening is provided on the gas tank, which includes a flange at its end. A gas filter device is fixed to the gas flange of the gas filling opening. The gas filter device includes a gas filter formed with a porous insulating material having pores with a diameter in a range from tens of micrometers to hundreds of micrometers. The gas filter has stability against the insulating gas and a decomposition product of the insulating gas.