PROBLEM TO BE SOLVED: To provide a plastic film bag with a gusset which prevents water leakage at a film heat-welded part in a part where the number of film layers changes when the part is subjected to pulling from the surrounding.SOLUTION: A bag body 10includes an opening part 13 opened into a quadrilateral shape at an upper top part, fold-in parts 12a, 12a respectively obtained by folding a side film inward to a predetermined depth, and a heat-welded seal part 16 which is located at a lower bottom part and is obtained by heat-welding an opening facing the opening part, and is made of a plastic film formed at a blow ratio in a range of 2.5 to 3.45 of a tubular film process. The heat-welded seal part 16 includes a side seal part 16a which extends from one side end to the inside for a prescribed length and a bending seal part 16b which is connected to the side seal and has an inclined side inclined to the side of the lower bottom part. The inclined side of the bending seal part intersects with a gusset part inner end 12', 12', and the intersecting part O is heat-welded.SELECTED DRAWING: Figure 1【課題】フィルム層数が変化する部分におけるフィルム熱溶着部分が周囲からの牽引を受けても水漏れしないマチ付きプラスチックフィルム袋を提供する。【解決手段】上方頂部に略四角形に拡口される開口部１３と、側フィルムを内側へそれぞれ所定深さ折り込んだ折り込み部１２ａ、１２ａと、下方底部にあって前記開口部に対向する開口を熱溶着した熱溶着シール部１６を備え、インフレーション法のブロー比が２．５～３．４５の範囲で作製されたプラスチックフィルムからなる袋体１００であって、熱溶着シール部１６は、一側端から内側へ所定長さ延びたサイドシール部１６ａと、サイドシールに繋がって前記下方底部側へ傾斜した傾斜辺を有する折曲シール部１６ｂを有し、折曲シール部の前記傾斜辺と前記折り込み部のマチ部内側端１２ａ'、１２ａ'とが交差し、交差した箇所Ｏが熱溶着されている。【選択図】図１
