<P>PROBLEM TO BE SOLVED: To solve the problem that it becomes difficult to perform phase control over a delay interferometer if input signal light is abnormal in a configuration that DQPSK modulated signal light is decoded by the delay interferometer and a data reproducing circuit. <P>SOLUTION: Disclosed is an optical receiver which branches an input light demodulated by DQSPK format and inputs them to first and second branching units each having a delay interferometer, a photoelectric converter, and a data reproducing circuit, wherein a first phase shifting means of adjusting the phase of a first delay interferometer is controlled based upon a first demodulated electric signal output from a photoelectric converter of the first branching unit and a second reproduced electric signal output from a data reproducing circuit of the second branching unit, a second phase shifting means of adjusting the phase of a second delay interferometer is controlled based upon a second demodulated electric signal output from a photoelectric converter of the second branching unit and a first reproduced electric signal output from a data reproducing circuit of the first branching unit, and abnormality of the input signal light is detected based upon a synchronous detection state of the first demodulated electric signal and first reproduced electric signal and a synchronous detection state of the second demodulated electric signal and second reproduced electric signal. <P>COPYRIGHT: (C)2009,JPO&INPIT 
OPTICAL RECEIVER AND ABNORMALITY DETECTION CONTROL METHOD