An optical receiving apparatus comprising: a combining unit for combining signal light and reference light; an optoelectric converting unit for converting, into electrical signals, two or more optical signals that enable reconstruction of a complex electric field signal of the signal light obtained by the combining unit; a sampling clock generating unit for generating a sampling clock signal that has a frequency (Fs) preset based on a symbol rate (Fc) of the signal light, wherein the sampling clock signal is asynchronous with the signal light; a digital converting unit (151, 152) for sampling at the frequency (Fs) of the sampling clock signal, an electrical signal obtained by the optoelectric converting unit and for converting the electrical signal into a digital signal; and a digital signal processing unit for demodulating a received signal based on a complex digital signal obtained from the digital signal obtained by the digital converting unit, including: a detecting unit for detecting a phase difference between the symbol rate (Fc) of the signal light and the sampling clock signal, and a sampling unit for sampling, based on the phase difference detected by the detecting unit, the complex digital signal at a frequency that is an integral multiple of a half of a sampling rate, wherein the digital signal processing unit is adapted to use the complex digital signal sampled by the sampling unit to demodulate the received signal, and characterised in that the sampling unit includes a decimation/interpolation unit for thinning out a signal from the complex digital signal or for inserting a signal into the complex digital signal so as to perform sampling at a frequency (Fs) that is an integral multiple of a half of a symbol rate (Fc) of the signal light, wherein the decimation/interpolation unit includes: a calculating unit for calculating a frequency (Fc/2xN) of the signal light based on the phase difference detected by the detecting unit and the frequency (Fs) of the sampling clock signal, and a flip flop circuit for performing retiming with respect to the complex digital signal at a frequency (Fc/2xN) calculated by the calculating unit. a combining unit for combining signal light and reference light; an optoelectric converting unit for converting, into electrical signals, two or more optical signals that enable reconstruction of a complex electric field signal of the signal light obtained by the combining unit; a sampling clock generating unit for generating a sampling clock signal that has a frequency (Fs) preset based on a symbol rate (Fc) of the signal light, wherein the sampling clock signal is asynchronous with the signal light; a digital converting unit (151, 152) for sampling at the frequency (Fs) of the sampling clock signal, an electrical signal obtained by the optoelectric converting unit and for converting the electrical signal into a digital signal; and a digital signal processing unit for demodulating a received signal based on a complex digital signal obtained from the digital signal obtained by the digital converting unit, including: a detecting unit for detecting a phase difference between the symbol rate (Fc) of the signal light and the sampling clock signal, and a sampling unit for sampling, based on the phase difference detected by the detecting unit, the complex digital signal at a frequency that is an integral multiple of a half of a sampling rate, wherein a detecting unit for detecting a phase difference between the symbol rate (Fc) of the signal light and the sampling clock signal, and a sampling unit for sampling, based on the phase difference detected by the detecting unit, the complex digital signal at a frequency that is an integral multiple of a half of a sampling rate, wherein the digital signal processing unit is adapted to use the complex digital signal sampled by the sampling unit to demodulate the received signal, and characterised in that the sampling unit includes a decimation/interpolation unit for thinning out a signal from the complex digital signal or for inserting a signal into the complex digital signal so as to perform sampling at a frequency (Fs) that is an integral multiple of a half of a symbol rate (Fc) of the signal light, wherein the decimation/interpolation unit includes: a calculating unit for calculating a frequency (Fc/2xN) of the signal light based on the phase difference detected by the detecting unit and the frequency (Fs) of the sampling clock signal, and a flip flop circuit for performing retiming with respect to the complex digital signal at a frequency (Fc/2xN) calculated by the calculating unit. a calculating unit for calculating a frequency (Fc/2xN) of the signal light based on the phase difference detected by the detecting unit and the frequency (Fs) of the sampling clock signal, and a flip flop circuit for performing retiming with respect to the complex digital signal at a frequency (Fc/2xN) calculated by the calculating unit.