A gas-permeable plug is adapted to be inserted in an insertion hole formed through a part of a casing in an inward-outward direction. The gas-permeable plug has a plug body and a permeable sheet including therein countless minute permeable pores. The plug body is injection molded from a material having rubber-like elasticity and the permeable sheet is integrally bonded to a sheet bonding portion by an anchor operation such that when the plug body is injection molded, at the sheet bonding portion, a part of a heat-melted liquid resin enters the minute permeable pores and/or the permeable gaps in irregular directions by the injection molding pressure and is solidified.