A relation between a light input power monitor value of an optical
transmission signal before passing through a fiber and an input signal
amplitude monitor value is recorded in advance in a storage device. Next,
actual optical-transmission-waveform is inputted into an optical receiver
module, and then comparisons between a light input power monitor value
and an input signal amplitude monitor value, and respective monitor
values in the case without having the waveform distortion as described
above are performed in an operation device to calculate a waveform
distortion value. According to the waveform distortion level calculated
herein, an optimum threshold value and an optimum phase adjusting value,
at which receiver sensitivity is maximized, are calculated in the
operation device to control a threshold-value adjusting circuit and a
phase-value adjusting circuit, thereby a threshold value and a phase
value that are optimum for an input distortion level can be established.