At least two melt-bondable sheets made of at least one material selected from plastic mesh, woven fabric, knitted fabric and non-woven fabric are stacked together, and the stacked sheets are melt-bonded and joined together such that a ridge is formed along a melt-bonded portion on a lateral side of the melt-bonded portion, the ridge being formed of a part of a resin material that is melted during pressurizing and melt-bonding and is solidified.
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