Abstract: A tunnel furnace height detector used for detecting ceiling heights of a tunnel furnace. The detector includes a plurality of detecting units arranged on a trolley traveling in the tunnel furnace in a manner such that their heights are progressively lowered by predetermined distances. Each of the detecting units includes a support base, and a solid piece pivotally connected to the support base automatically returnable from a pivotally moved position forced by an external force to an original position. The solid piece has one end in contact with a lowered ceiling and the other end closing and opening a measuring circuit system forming a closed circuit.