To provide a jitter measuring device which measures the jitter contained in a periodic signal at real time.

The reference signal emitted from a reference signal generator 1 is branched, with one inputted in an object 2 which is to be measured while the other in a signal delay device 3. The object 2 outputs a measured signal 4a of the same frequency as the inputted reference signal to an A/D converter 4. The signal delay device 3 outputs the inputted signal added with a specified delay amount as a clock signal 4b to the A/D converter 4. Since the delay amount is always constant through measurement, the quantization of input signal by an A/D converter is performed periodically through measurement. The A/D converter 4 quantizes the inputted measured signal 4a at such timing as based on the clock signal 4b, with digitized data outputted to a memory 5. The memory 5 memorizes the data outputted from the A/D converter 4. A calculator 6 calculates a jitter amount based on the data accumulated in the memory 5.