The present invention makes it possible (1) to produce a natural rubber wet master batch and/or a synthetic isoprene rubber wet master batch by mixing a slurry solution of biodegraded rubber powder dispersed in water with a natural rubber latex solution and/or a synthetic isoprene rubber latex solution. Further, the present invention makes it possible (2) to produce a natural rubber wet master batch and/or a synthetic isoprene rubber wet master rubber powder with a natural rubber latex solution and/or a synthetic isoprene batch by mixing a slurry solution of dispersed carbon black and/or silica and rubber latex solution. A rubber composition having high physical properties can be obtained by adding a natural rubber master batch and/or a synthetic isoprene rubber master batch obtained by drying the natural rubber wet master batch and/or the synthetic isoprene rubber wet master batch. Further, the rubber composition containing the wet master batch obtained by the method in combination of above (1) and (2) also exhibits high physical properties.