To provide a duct burner apparatus and a waste heat recovery boiler equipped with the duct burner wherein the number of installation of burner units is reduced without a heat transfer surface and a duct wall of a heat exchanger located downstream the duct burner apparatus becoming locally high temperature.

Gas passing through (an opening) between baffles 8A and 8B and a burner element 12 is bent from the opening to a backstream side of the baffles 8A and 8B by moving the baffles 8A and 8B to a frontstream side of the burner element 12. Since the gas passing through between the baffles 8A and 8B and the burner element 12 is bent to the backstream side of the baffles 8A and 8B, the size of a circulation flow is increased on the backstream side of the burner element 12 but decreased at the backstream side of the baffles 8A and 8B. Further, the shape of the circulation flow on the backstream side of the burner element 12 is spread largely in a height direction of a cross section of a duct 11 with the length toward the backstream side relatively shortened. A fuel injected from a fuel nozzle is well mixed with contact and stirred with waste gas owing to the circulation flow on te backstream side of the burner element 12 for complete combustion by effectively making used of residual oxygen in the waste gas.