To provide a method capable of stably manufacturing a plastic molded product of good quality from a waste plastic.

In this refining treatment method of the waste plastic, the waste plastic 103 is charged in a first extruder 10 equipped with a vacuum means to be held to a vacuum state while heated to be melted. After the molten plastic is subjected to filtering treatment, the filtered melt is cooled and solidified to obtain a granular product 105 of the waste plastic. The granular product 105 is charged in a second extruder 20 equipped with a vacuum means and melted while held to a vacuum state under heating. Then, the obtained melt is subjected to extrusion molding.