step 1 of forming small pores in a surface of a base made of a plastic to produce a base with small pores; step 2 of immersing the base with small pores, obtained in step 1, in a solution including a foaming agent to prepare a foaming agent-including base; step 3 of immersing the foaming agent-including base, obtained in step 2, in a foaming solution that makes the plastic swell and the foaming ag