BIAXIALLY ORIENTED POLYESTER FILM

To provide a biaxially oriented polyester film that is excellent in runnability and take-up performance, and, furthermore, is excellent in dimensional stability, and, when used as a magnetic recording medium, that produces a high-density magnetic recording medium having a smooth magnetic layer as well as having a small dimensional change due to the environmental change in temperature or humidity, and the storage, and having a low error rate.SOLUTION: The problem is solved by a biaxially oriented polyester film in which the humidity-expansion coefficient in film width direction is 4.5 to 6.5 ppm/%RH, the temperature-expansion coefficient in the width direction is -3 to 7 ppm/°C, and, on at least one outermost layer surface, the number of protrusions with a height of 60 nm or more is 0 to 200/mm2.SELECTED DRAWING: None 【課題】走行性や巻き取り性に優れ、さらには寸法安定性に優れ、磁気記録媒体とした際に平滑な磁性層を有すると共に温度や湿度の環境変化や保存による寸法変化が小さい、エラーレートの少ない高密度磁気記録媒体となる二軸配向ポリエステルフィルムを提供すること。【解決手段】フィルムの幅方向の湿度膨張係数が４．５〜６．５ｐｐｍ／％ＲＨであり、幅方向の温度膨張係数が−３〜７ｐｐｍ／℃であり、少なくとも一方の最外層表面において、高さ６０ｎｍ以上の突起個数が０〜２００個／ｍｍ２である二軸配向ポリエステルフィルムとする。【選択図】なし