To insert various measuring means in a furnace without stopping the combustion of a burner and with inflow of the atmosphere into the furnace preventing from occurring.

The central axis of a cylindrical furnace body 2 is extending in a horizontal direction and rotatably arranged, a burner port 3 is arranged on one side of the furnace body 2 and an exhaust port 5 on the other side thereof. In a so formed rotation type melting furnace, a through-port 11 through which a measuring means 10, such as a temperature sensor and a sample sampler, is inserted in the furnace is formed in a furnace cover 7 to cover the burner port 3 or in an exhaust duct 8 arranged in a manner to cover the exhaust port 5.