This is the "Breast Line Cover", and it's here to block unwanted leering.
Priced at ¥1,050 ($13) and available in six colors, the Breast Line Cover is for those women who want wear stylish clothing, but want to avoid embarrassing "accidental glimpses of cleavage".
The Breast Line Cover resembles a camisole, but simply affixes itself to the wearer's bra and can offer complex protection that camisoles simply cannot. Yes, it looks like a bra thong.
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And those worried about people peeping at your underpants? Japan has you covered. Literally.
キャミ要らず！"谷間チラリ"を防ぐ「胸元ラインカバー」が便利 [Walkerplus via 痛いニュース]