A feedstock granular substance (7) is fed into a cylindrical retention tube (3) provided with a multiplicity of retention parts (8) for retaining a granular substance (7) moving along the inner circumference thereof. In the state of having the feedstock granular substance (7) retained by the retention parts (8) of the retention tube (3), a grinding device (4) having multiple flexible long grinding materials (6) radially extending from the rotational axis extending along the axial center of the retention tube (3) is rotated to thereby realize rotation with the distal ends of the long grinding materials (6) brought into contact with the surface of the granular substance (7) and surface treats the granular substance (7). Thus, production of a surface-treated granular substance is attained.