Optical waveform observing apparatus including a sampling streak tube to which is applied an incident light beam having a waveform of repetitive frequency 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 deflecting trigger signal is generated by a time sweep circuit that stepwise delays the occurrence of that signal when the sampling operation is carried out a predetermined number of times. An integration circuit integrates the output of the streak tube for the predetermined number of sampling operations for a sampling time and is then reset to begin integrating the streak tube output for the predetermined number of sampling operations for a next sampling time. The integration circuit output is digitized and displayed.