Filters for the supply of inert gas to packing machines are sterilized by passing steam and sterile air through the filters via a number of ducts, valves and governors. A simpler method for achieving the sterilization is obtained by sterilizing the filter (11) jointing with the packing machine which may be done by connecting one side of the filter in series to the circulation system of the machine for sterilization by means of chemical sterilizing agent. In this way the abovementioned arrangement is simplified considerably in that a large number of valves and ducts becomes superfluous.