To provide a method for manufacturing efficiently a solid feedstock and fuel of a homogeneous calorific value.

The method comprises charging into the inside of a heating furnace 20 a mixture of a waste plastics 10 along with such a partial combustion slug formed in a partial combustion furnace of the heating furnace 20, an unburnt material of petroleum coke, fly-ash or heavy-oil ash as a fusion-preventing agent for preventing the fusion of waste plastics 10 onto the furnace wall of the heating furnace 20 or that among waste plastics 10 one another, in a weight ratio of the waste plastics of 100 pts.wt. relative to 5-200 pts.wt. of the partial combustion slug, unburnt material of petroleum coke, fly-ash or heavy-oil ash.