A signal detection method used in an optical receiver apparatus detects the variation of an optical input level from the presence or absence of a clock signal and appropriately controls a dispersion compensator, thereby enabling the presence or absence of an input signal to be correctly determined. The signal detection method includes: detecting the level of input light of an optical amplifier, storing the level of the detected input light, comparing the level of the stored previous input light with the level of current input light, detecting the level variation of the input light by the comparison to detect the state change of the presence or absence of an optical signal, performing a dispersion compensation on the input light, and extracting a clock from an optical input. When the level variation of the input light is detected, the presence or absence of the optical signal of the input light is determined from the presence or absence of the clock signal. When the clock signal is not extracted, the dispersion amount is swept.