To suppress the puffing of the subject feed, raise the specific gravity, facilitate the sedimenting in water and prevent the subject feed from deteriorating the moldability in granulation by controlling the hydration ratio based on a raw material when granulating the raw material with an extruder.

The hydration ratio based on a raw material is regulated to 15-20% when carrying out the granulation with an extruder. The raw material is granulated at 15-20% hydration ratio by using a die nozzle manufactured from brass, gunmetal or a plastic in a die of the extruder apparatus. A plastic selected from a polyamide-imide, a polyacetal, a phenol resin, polypropylene and polycarbonate is preferred as the plastic and the superheat-resistant polyamide-imide is preferred from the viewpoint of durability. When the hydration ratio is regulated to 15-20%, the sedimentation ratio of the objective feed is 70-80%.