To provide a decomposition apparatus for waste plastics which has a reaction tank using a furnace body to avoid any problem concerning the strength of a residue-exhaust structure.

A decomposition apparatus for waste plastics, which possesses a decomposition tank 2 containing a furnace body 1 in a cylindrical form and a revolving shaft 12 surrounded by a stirring means 13 which revolves around the central axis of the furnace body, is improved by establishing an umbrella form slope 14 which slopes down to the periphery and also a circulatory tube which sucks up a liquid-phase polymer inside the furnace body and discharges it onto the umbrella form slope, and by establishing a scraper 18, established along the umbrella form slope in the direction of the slope, adjacent to an exhaust means 9, which transports solid matters, to scrape off solid residues contained in the liquid-phase polymer discharged onto the umbrella form slope via the circulatory tube and to discharge the recovered residues outside via the exhaust means.