PURPOSE: To eliminate the floating-up of the sponge ball bue to imperfect air discharge during compression process of the sponge ball by a method wherein the sponge ball is past through a certain gap formed between a pair of rollers rotating in the same direction of rotation with different peripheral speeds.

CONSTITUTION: A drum, which is filled with liquid 6 therewithin and respectively provided with a charging port 2 and a discharging port 3 of sponge balls at the upper and lower parts thereof, and pair of the rollers 4 and 5, which serve to discharge the air in the sponge ball 7 by compressing the sponge ball 7 within the liquid 6. The rollers 4 and 5 are rotated in the same direction of rotation with different peripheral speeds by means of an intermediate gear 10. Owing to the method as described above, the time during which the sponge ball is kept under compression can be markedly prolonged and at the same time the number of rotation of the sponge ball under compression is increased, resulting in enabling to discharge the air perfectly. Consequently, the satifactory absorption of water can be accomplished and the elimination of the floating-up phenomenon of the sponge ball can be resulted.