a pulse generating section having first pulse generating means for detecting edges of said data-signal-under-measurement to output a first pulse signal having a pulse width set in advance corresponding to said edge and second pulse generating means for detecting boundaries of data sections where data values do not change in said data-signal-under-measurement to output a second pulse signal having a pulse width set in advance corresponding to timing of the boundaries of said detected data sections at each of the boundaries of the data sections at each of the boundaries of the data sections of the data-signal-under-measurement; an integrator section for outputting a jitter measuring signal whose signal level increases at a predetermined rate of increment during when the signal outputted from said pulse generating section indicates a first logic value and whose signal level decreases at a predetermined rate of decrement during when the signal outputted from said pulse generating section indicates a second logic value which is different from said first logic value; and

a jitter calculating section for calculating timing jitter in said data-signal-under-measurement based on said first pulse signal, second pulse signal and on each of said signal level of said jitter measuring signal outputted from said integrator section at the timing of edge of said pulse signals.