A decoding device that decodes demodulated data obtained by demodulating a quadrature modulated signal arising from digital modulation of a carrier and detects synchronization, the decoding device includes, a decoder configured to decode first demodulated data that is the demodulated data obtained by demodulating the quadrature modulated signal and is composed of in-phase axis data and quadrature axis data. The decoding device decodes second demodulated data obtained by interchanging the in-phase axis data and the quadrature axis data of the first demodulated data. A synchronization detector is configured to detect a boundary between predetermined information symbol sequences from first decoded data obtained by decoding the first demodulated data and detect the boundary from second decoded data obtained by decoding the second demodulated data. The synchronization detector selects and outputs one of the first decoded data and the second decoded data based on a result of the detection of the boundary.