A quadrature demodulation device effectively utilizes the result of quadrature detection in order to improve demodulation efficiency. The quadrature modulation device includes a quadrature detector DT which performs quadrature detection on a reception signal obtained by receiving a radio wave from a radio frequency identification tag, to generate an in-phase signal and a quadrature-phase signal, and a demodulation circuit DM which detects a preamble of a particular transition pattern in at least one of the in-phase signal and the quadrature-phase signal and which decodes the at least one of the in-phase signal and the quadrature-phase signal to obtain data succeeding the preamble, the demodulation circuit DM further performing error detection on a decoding result to check for errors.