A license manager running on a central-management server receives a license request for licensing a managed node as a client of a central-management product running on a central-management server. A discharge tube 2 is heated during the first period for supplying a large amount of electric power to discharge tube 2 to elevate the tube temperature and thereby uniformly disperse mercury vapor in discharge tube 2. The first filtering element and the second filtering element comprise filtering elements of high accuracy, with a steep roll-off but poor power handling capabilities.