PURPOSE: To hold excellent air-permeability and stabilized compression strength by molding the composition for an open cell foam of olefin-based resin into a sheet-shape, heating and foaming the sheet-shape and cooling it and continuously compressing the obtained open cell foam of olefin-based resin at least constant compressibility with two pieces of rolls.

CONSTITUTION: An open cell foam of olefin-based resin is continuously compressed at ≥50% compressibility with two pieces of rolls. As the number of times for compression and rolling is made plenty, the continuity of a bubble in the direction of thickness is raised. The effect after the second time is made small and 1-5 times are preferable but 1-2 times are made the most preferable when considering the economic aspect. As compressibility, the effect for improving the degree of communication in the direction of thickness is made small at ≤50% compressibility and it is preferably regulated to 70-90%. The open cell foam 4 is passed through the guide rolls 1 and compressed in the direction of thickness between two rolls 2, 2' for compression and thereafter wound via the draw-off rolls 3.