A metal melting furnace equipped with a preheating tower having a plurality of fork-shaped shelf-plates extending into the inside thereof, comprising a gas mixing chamber provided in the lower end portion of the tower, communicating with the upper portion of the melting furnace, a duct for circulating combustion exhaust gas from the top portion of the tower into the gas mixing chamber, a damper arranged in the duct for controlling the amount of the combustion exhaust gas circulated into the gas mixing chamber, and a plurality of nozzles connected to the duct and opening into the gas mixing chamber.