Method of removing impurities from plastic

A method of removing impurities from plastic, the method comprising:

• cleaning the plastic by spraying the plastic to remove impurities with superheated steam having a temperature of 180° C. to 300° C. and a relative dielectric constant of 40 or less followed by lowering the temperature of the superheated steam to 150° C. in a superheated steam atmosphere to obtain a cleaned plastic, wherein
• the plastic is polyolefin,
• the plastic is a used plastic molded product,
• the plastic is not decomposed by the superheated steam,
• the impurities are dissolved in the plastic,
• the superheated steam is sprayed to the plastic under a condition of a non-closed system where the superheated steam is supplied and sequentially discharged.