Abstract: Optical waveform observing apparatus including a sampling streak tube to which is applied an incident light beam having a waveform to be observed. An electron beam corresponding to the incident light beam is repetitively deflected in the streak tube, in response to a repetitive deflecting trigger signal, to sample the electron beam. The repetitive deflection of the electron beam is periodically stopped for a first time period. An integration circuit integrates data outputted by the streak tube. A subtraction circuit subtracts the integration of streak tube data outputted during the first time period from the integration of streak tube data outputted during a second time period when the repetitive beam deflection is not stopped, so that background noise and dark currents are not included in the subtraction result.