PURPOSE: To easily execute a solidifying treatment having high safety without a possibility of eluting of harmful metals contained in waste by press kneading a mixture obtd. by mixing waste plastic materials and waste at prescribed ratios to soften and melt the waste plastic materials, the molding the mixture to a solidified body of a prescribed shape.

CONSTITUTION: A mixing means mixes the waste plastic materials and the waste to be subjected to the solidification treatment at the prescribed ratios. Press kneading means 1, 8 soften and melt the waste plastic materials by press kneading the mixture obtd. by the mixing means. Further, molding means 13a, 13b mold the molten mixture obtd. by the press kneading means to the solidified body of the prescribed shape. At least ≥2 units of the press kneading means 1, 8 are disposed. Further, the press kneading means 1, 8 are provided with rotating in directions opposite to each other while meshing each other. The press kneading means are provided with heaters 7, 14 near the molten mixture discharge ports of these means.