A powder rubber obtained by crushing a waste rubber comprising various kinds of rubbers is subjected to treatment of making unevenness on a surface thereof. A method in which a microorganism growing with a rubber being an only carbon source and powder rubber having an average particle diameter of 1 mm or less are added to a culture medium to cultivate the microorganism can be applied as treatment of making the unevenness. The powder rubber is decomposed by this method, and the surface area is increased, whereby a rubber composition which is excellent in abrasion resistance, fracture characteristics and crack growth resistance can be obtained. A WU-YS05 strain (Patent Microorganism Depository Center: Accession number: FERM P-18660) belonging to Pseudonocardiaceae is effective as the microorganism.