To obtain a wooden material-containing thermoplastic resin compsn. capable of being formed into a granular product having a uniform shape.

A preparatory mixing process obtaining a premix by a first mixing process stirring and mixing a thermoplastic resin compsn. and cellulosic crushed matter by a container fixing type heating mixer fitted with a heating medium jacket to perform softening mixing while raising the temp. of the resulting mixture to temp. equal to or higher than the softening temp. (melting temp.) of the thermoplastic resin compsn. or higher, a second mixing process cooling and mixing the mixture softened and mixed in the first mixing process by a container fixing type cooling mixer fitted with a cooling medium jacket and a granular product manufacturing process by a temp regulable granulator 4 quantitatively supplying the premix obtained by the preparatory mixing process to the raw material supply part 3 of a continuos two-roll type kneader and passing the same through the nip between both rolls 1, 2 to melt and knead the same are provided.