According to one embodiment, a decoder includes a control unit and a decoding unit. The control unit determines a window size applied to a first target frame to be a first value and determines a window size applied to a second target frame different from the first target frame to be a second value different from the first value. The decoding unit carries out windowed decoding of a spatially coupled code on the first target frame with the window size set to the first value and carries out windowed decoding of a spatially coupled code on the second target frame with the window size set to the second value.