According to one embodiment, a decoder includes: an error detecting device detecting a fact that an error is included in an encoded bitstream, the error making it impossible to predict a pixel value using a prediction mode; an error processing device replacing the prediction mode ruled in the bitstream with a prediction mode having a prediction direction most close to a reference prediction direction for a plurality of prediction modes allowing predicting the pixel value; and a prediction processing device predicting the pixel value using the prediction mode replaced by the error processing device.