Semi-circular cover members 17 and 17 having a spring function for opening and closing the openings of pores 15S are provided on the cavity inner wall side of a porous air vent 15 in order to close the openings of the above pores 15S while a small space remains between the above cover members 17 and 17 and the openings of the pores 15S of the porous air vent during vulcanization, whereby the occurrence of the occlusion of the above pores 15S is reduced and stains adhered to the cavity inner wall side of the mold 10 are removed by physical means such as a blast treatment while the above cover members 17 and 17 are closed at the time of cleaning to remove the stains of the above mold 10. Further, the above openings are opened as required to allow a cleaning liquid 24 go into the above pores 15S to clean the above mold 10 and remove stains in the pores 15S.