A sampling optical pulse generation unit emits a sampling optical pulse. The sampling optical pulse has a period different from a predetermined offset time to integral multiplication of a clock period of a data signal, and the data signal modulates an optical signal of a monitoring target. An optical sampling unit performs sampling of the optical signal of the monitoring target using an electroabsorption optical modulator. The electroabsorption optical modulator has a characteristic in which an absorption index is changed with respect to light propagating through an optical path according to a level of an electric field. The optical signal of the monitoring target is input into one of two optical terminals of the electroabsorption optical modulator. The sampling optical pulse is input into another of the two optical terminals through a photocoupler. The absorption index with respect to the optical signal of the monitoring target is lowered by generating a mutual absorption saturation characteristic of the electroabsorption optical modulator when the sampling optical pulse is supplied, and the sampling of the optical signal of the monitoring target is performed to output an optical pulse signal from the other of the two optical terminals through the photocoupler.