To prevent a bed material from being penetrated into the differential pressure detecting hole of a fluidized bed differential pressure detecting device by providing a separation box for separating the particle of the bed material from a fluid and carrying only the fluid to the differential pressure detecting hole on the fluidized bed layer of the differential pressure detecting hole.
A box-shaped separation box 3 is connected to the inside of a boiler furnace wall 1 so as to enclose a fluidized bed differential pressure detecting hole 2 within a boiler body. The bed particle contained in a flowing medium to be penetrated into the separation box 3 causes a block (bridge or arching) in a bottleneck-like narrow angle part 4 having a small opening width. The bridge prevents the bed particle exceeding it from being penetrated into the separation box 3. On the other hand, the fluid in the flowing medium gets into the differential detecting hole 2 in the deep part of the separation box 3 and reaches a differential pressure detecting device, whereby the differential pressure within the boiler body is detected. Since the penetration of the bed material particle to the differential pressure detecting hole 2 can be prevented, and the removing work of the particle can be dispensed with.