PURPOSE: To reduce a manufacturing cost and a material cost as well as to attain uniformity of the density of a product and to stabilize the sound absorbing characteristics of the product, by inserting a number of wooden chips or powder of cement, ceramic and so on in a cavity of a rubber mold and pressing from the outside of the rubber mold.

CONSTITUTION: After a cap 9 placed on the upper end of a rubber mold 5 is removed, a number of wooden chips 10 or powder of cement, ceramic and so on are spread in a cavity 7 surrounding a core metal 4. Then, after the cap 9 is fitted on the upper end of the rubber mold 5, the rubber mold 5 is inserted in a rubber press main body 1, which is then lidded with a top cap 2. The rubber mold 5 is then pressed under oil or air pressure up to a specified pressure, e.g. 500 kg/cm2. After keeping the pressure for a specified pressure, the pressure is lowered and after the pressure is completely out, the rubber mold 5 is taken out from the main body 1. Then, a compressed and shrunk wooden material is taken out from the rubber mold 5 and used as a sound absorbing material 12. The manufacturing cost is thereby reduced especially when a wooden material is used as a starting material. If a thinned out material is used, the manufacturing cost if further reduced.