An optical signal sampling apparatus for high-speed gating of an optical signal in the form of repetitive waveforms comprises an input optical path for introducing the optical signal to an optical amplifier; the optical amplifier whose gain is adjustable by a gating electrical signal, for amplifying the optical signal; a driving circuit for providing, in response to a sampling signal, the optical amplifier with the gating electrical signal having a series of pulses which are gradually shifted on a time axis with respect to the repetitive waveforms of the optical signal; and an output optical path for introducing an output optical signal from the optical amplifier.