Abstract: A heat exchanger for flue gas in which water tubes are inserted through the flue wall into the flue from the exterior thereof so as to be adjustable in the depth of insertion. Each of the water tubes has a structure comprising an inner water feed tube and an outer recovering tube for recovery of the waste heat by returning the water from the water feed tube to generate steam by heat transferring from the waste gas passing through the flue. Headers are provided at each exterior top of the feed water tubes and recovering tubes and are connected respectively to a water-steam drum through movable connecting tubes. The headers are movable in co-operation with driving means provided at the exterior of the flue so as to adjust the heating surface of the water tubes inserted into the flue by the action of the driving means.