An electroabsorption optical modulator used as a sampling element has two optical terminals and a power supply terminal, the two optical terminals are used to input and output light, and the power supply terminal imparts an electric field to an optical path connecting the two optical terminals. The electroabsorption optical modulator has a characteristic in which an absorption index is changed with respect to the light beam propagating through the optical path according to a level of the electric field. A monitoring (synchronization sampling) object optical signal is input into one of the two optical terminals of the electroabsorption optical modulator. A predetermined direct-current voltage exhibiting a high absorption index for the monitoring (synchronization sampling) object optical signal is imparted to the power supply terminal of the electroabsorption optical modulator. A sampling optical pulse is input into another of the two optical terminals of the electroabsorption optical modulator through a photocoupler. The absorption index for the monitoring (synchronization sampling) object optical signal is lowered by a mutual absorption saturation characteristic of the electroabsorption optical modulator, and synchronization sampling of the monitoring (synchronization sampling) object optical signal is performed to supply an optical signal from the other of the two optical terminals of the electroabsorption optical modulator through the photocoupler. The mutual absorption saturation characteristic is generated when the sampling optical pulse is input into the other of the two optical terminals.