The present invention relates to an electro-optic sampling oscilloscope. The delay circuit in the electro-optic sampling oscilloscope comprises a delay time detecting circuit, a regulation time determining circuit, a counter circuit and a delay regulating circuit. The delay time detecting circuit detects in the trigger signal a value corresponding to the delay time of a reference clock from a reference clock generating circuit. The regulation time determining circuit determines a regulation time based on the value detected by the delay time detecting circuit so that the regulation time is an integer multiple of the reference clock. The counter circuit is triggered by the trigger signal to count the reference clock through a specific value. The delay regulating circuit employs a signal related to the regulation time from the regulation time determining circuit, to delay the signal output from the counter circuit by the regulation time.