According to one embodiment, a decoder device includes a decoder configured to decode a to-be-decoded sequence by performing an iterative decoding process and to perform a parity check of a decoding result using a check matrix, a detector configured to detect that the to-be-decoded sequence is a non-code word based on a parity check result for each row of the check matrix by the decoder, and a controller configured to control the decoder according to a detection result of the detector.