To provide a mounting structure of a dummy tube in a layer of a fluidized bed boiler which enables accurate prevention of the slipping off of the dummy tube.

One end of a dummy tube 46 is clamped with a heat exchanger tube 9 by a first matching metal fitting 48 being sandwiched from above and below it while a stopper 57 for constraining the axial movement of the dummy tube 46 with respect to the first matching metal fitting 48 is fastened on the dummy tube 46 so as to sandwich the first matching metal fitting 48 from before and behind it. The other end part of the dummy tube 46 is clamped with the heat exchanger tube 9 by a second matching metal fitting 48' being sandwiched from below and above it. The other end part of the dummy tube 46 not clamped by the matching metal fitting 48' is placed on a U-shaped support metal fitting 58 fastened on the second matching metal fitting 48' being inserted therethrough while a stopper 59 is fastened on the other end part of the dummy tube 46 placed on the support metal fitting 58 being inserted therethrough with a specified space given before and after the support metal fitting 58.