To provide a plastic melt auxiliary unit which enables rapid reduction of a large quantity of waste plastic to oil by increasing the melt capacity through effective heating of the waste plastic.

This melt auxiliary unit 10 has an inner pipe 12 which introduces waste plastic to a melt vessel 20 of a waste plastic liquefaction unit. At the outer circumference of the inner pipe is an outer pipe 13 which forms an outer duct M2. At the outer circumference of the outer pipe 13 is an insulating material 14 which forms a heat insulating room S. When waste plastics are heat treated, solid waste plastic is introduced through a plastic conduit 15 to the duct M1, and, at the same time, each predetermined quantities of a gaseous inner heat medium oil and a liquid inner heat medium oil are supplied through inner heat medium oil conduits 16a and 16b. On the other hand, the outer heat medium oil is circulated at a fixed rate in the outer duct M2.