To detect a liquid surface of a melt-plastic in a pyrolysis vessel for pyrolizing a plastic, and to prevent a danger by an abnormal arising of the liquid surface.

A pressure in a detecting pipe 4 and a pressure in a pyrolysis vessel 2 are detected by pressure detectors 5 and 6, respectively, wherein the detector 4 having a closed upper end and a opened lower end is extended from an upper part to lower inside of the pyrolysis vessel 2. When the liquid surface of the melt-plastic arises abnormally and gets in the lower end of the detector 4, the elevation of the liquid surface is detected from a pressure difference by the rise of its inner pressure.