In recent years, environmental pollution in the vicinity of final disposal sites for inert waste has resulted in problems such as deterioration of water quality and increases in leachates and odours. Accordingly, the number of sites that no longer accept these wastes has risen. One of the main causes of these problems is represented by organic matter adhering to waste plastics, accounting for the majority of waste deposited at final disposal sites for inert waste. Furthermore, the amount of waste plastics undergoing thermal recycling has increased owing to recent energy demands. This study examines the recycling of waste plastics into solid fuel, known as refuse paper, and plastic fuel (RPF), by focusing on the washing method. The first washing effect at liquid to solid ratio (L/S) = 1 was confirmed to be effective in the rough removal of adherent organic matter and was also found to be suitable for pretreatment for the second washing for thermal recycling. As compared with the first washing, although effectiveness of the second washing with regard to chemical oxygen demand (COD) was not observed, the effectiveness of the second washing versus total nitrogen (T-N) at low speed and L/S = 1 was comparable to that of the first washing at L/S = 10. These results indicate that a combination of first washing at L/S = 1 and second washing at low speed and L/S = 5 constituted an effective pretreatment for refuse paper and plastic fuel (RPF) production. Furthermore, a mixed antichlor was produced by combining a commercial antichlor and waste having a dechlorination effect as countermeasures against hydrogen chloride gas emitted when RPF derived from the burning of waste plastics. We tested its performance in reducing Cl gas generation by mixing it into the RPF. As the antichlor was added to RPF for low Cl gas emission, 36% of the CaO–oyster shells added as antichlor against Cl content was found to be effective in reducing Cl gas emission. Thus, it was concluded that using an appropriate percentage of antichlor, the combination of first and second washing processes (under some conditions) was effective in the pretreatment of waste plastic destined to be recycled as RPF.