1. A filter coefficient control apparatus, for controlling a coefficient of a filter of a phase recovering apparatus for an optical coherent receiver, comprising: a phase offset obtaining means, for obtaining a phase offset between a phase of a symbol received by the optical coherent receiver and a data modulation phase of the symbol; an autocorrelation calculating means, for calculating an autocorrelation and related statistics of the phase offset; and a filter coefficient determining means, for determining the coefficient of the filter in accordance with the autocorrelation and related statistics, wherein the autocorrelation calculating means includes: a delay sequence obtaining unit, for obtaining a delay sequence by differently delaying the phase offset; and an autocorrelation sequence obtaining unit, for determining the autocorrelation and related statistics of the phase offset in accordance with the delay sequence.