Anata no Torikago

One rainy day, Nakanishi Mina is abruptly kidnapped. When she wakes up, she's handcuffed to a bed in a mansion. In comes the butler, Shirayama Masaya, to explain that she had been abducted under the orders of the mansion's owner. The owner, Iwatsuki Kanata, hides his face with a mask. He keeps Mina handcuffed and rapes her, night after night... [Translated from DLsite]