[Solution] This invention is characterized in being provided with: a gas-permeable sheet that is permeable to gases; a holding body provided with a gas-channeling hole for channeling gas, said body holding the gas-permeable sheet in the gas-channeling hole; and a lid body attached to the holding body so as to cover the opening of the gas-channeling hole on the side on which the gas is discharged; a gas discharge path being formed between the lid body and the holding body, the gas discharge path discharging, from between the lid body and the holding body, the gas discharged from the gas-channeling hole.