A conventional method of pyrolytically decomposing plastic to make low-molecular compounds is poor in efficiency of use of energy due to high heating temperatures (at least 600 C. and 800 C. at maximum). The invention provides that a functional powder is heated to a predetermined temperature and pieces of plastic to be disposed are caused to be contacted with the heated powder, the functional powder including a powder which does in an aqueous solution state show alkalinity, and a photo-catalyst effect powder.