PURPOSE: To detect the height of fluidized layer while scarcely stopping the operation of the fluidized layer.

CONSTITUTION: The amount of air to be fed into the furnace 4 for fluidization is reduced to minimum to make the fluidized bed 5 to almost immobile condition, and the measuring pipe 1 is lowered to have its contact plate 3 to touch the surface of fluidized layer. And the position is read by the scale precalibrated on the measuring pipe 1. When the furnace 4 is at negative pressure, air passes through the measuring pipe 1 due to natural ventilation, thus preventing the measuring pipe 1 from being damaged by overheating. When the furnace 4 is at positive pressure, gas such as air or nitrogen is introduced through the measuring pipe 1 to protect it from being damaged by overheating. An adapter 4 is used to fix the measuring pipe 1. Since the measuring pipe 1 is used about once a month in an incinerator, it is usually raised at normal operation to an upper position and fixed with bolts 7.