An optical receiver for receiving differential coded phase-modulated signal light, comprising: a Mach-Zehnder interferometer for converting the phase-modulated signal light to intensity modulation signal light; a balanced optical receiver for photoelectrically converting the intensity modulation signal light from two output ports of the Mach-Zehnder interferometer to output a difference between converted electric signals; a minute modulation signal generating circuit for minutely modulating the transmittance of the Mach-Zehnder interferometer at a low frequency; a minute modulation signal detecting circuit for detecting a minute modulation signal included in the signal output from the balanced optical receiver; a transmittance detecting circuit for judging, from the signal output from the balanced optical receiver, whether the transmittance of the two output ports of the Mach-Zehnder interferometer is maximum or minimum at a carrier frequency of the phase-modulated signal light; a synchronous detection circuit for synchronously detecting the minute modulation signal included in the signal output from the balanced optical receiver with the signal generated from the minute modulation signal generating circuit; a switch for switching the minute modulation signal detecting circuit and the transmittance detecting circuit to connect it to the synchronous detection circuit; and a control circuit for detecting the signal output from the synchronous detection circuit as a shift amount between the frequency at which the transmittance of the Mach-Zehnder interferometer becomes maximum or minimum, and the carrier frequency to feed back the transmittance of the Mach-Zehnder interferometer to make the shift amount zero or a predetermined value, characterised in that: the switch is set to connect the transmittance detecting circuit to the synchronous detection circuit in an initial stage of frequency pull-in, and if the transmittance detecting circuit detects that a first output of the two outputs of the Mach-Zehnder interferometer is maximum and a second output of the two outputs is minimum, the connection of the switch is switched from the transmittance detecting circuit to the minute modulation signal detecting circuit. a Mach-Zehnder interferometer for converting the phase-modulated signal light to intensity modulation signal light; a balanced optical receiver for photoelectrically converting the intensity modulation signal light from two output ports of the Mach-Zehnder interferometer to output a difference between converted electric signals; a minute modulation signal generating circuit for minutely modulating the transmittance of the Mach-Zehnder interferometer at a low frequency; a minute modulation signal detecting circuit for detecting a minute modulation signal included in the signal output from the balanced optical receiver; a transmittance detecting circuit for judging, from the signal output from the balanced optical receiver, whether the transmittance of the two output ports of the Mach-Zehnder interferometer is maximum or minimum at a carrier frequency of the phase-modulated signal light; a synchronous detection circuit for synchronously detecting the minute modulation signal included in the signal output from the balanced optical receiver with the signal generated from the minute modulation signal generating circuit; a switch for switching the minute modulation signal detecting circuit and the transmittance detecting circuit to connect it to the synchronous detection circuit; and a control circuit for detecting the signal output from the synchronous detection circuit as a shift amount between the frequency at which the transmittance of the Mach-Zehnder interferometer becomes maximum or minimum, and the carrier frequency to feed back the transmittance of the Mach-Zehnder interferometer to make the shift amount zero or a predetermined value, characterised in that: the switch is set to connect the transmittance detecting circuit to the synchronous detection circuit in an initial stage of frequency pull-in, and if the transmittance detecting circuit detects that a first output of the two outputs of the Mach-Zehnder interferometer is maximum and a second output of the two outputs is minimum, the connection of the switch is switched from the transmittance detecting circuit to the minute modulation signal detecting circuit. the switch is set to connect the transmittance detecting circuit to the synchronous detection circuit in an initial stage of frequency pull-in, and if the transmittance detecting circuit detects that a first output of the two outputs of the Mach-Zehnder interferometer is maximum and a second output of the two outputs is minimum, the connection of the switch is switched from the transmittance detecting circuit to the minute modulation signal detecting circuit.