PURPOSE: To measure the jitter of the signal, of which frequency band is limited, by detecting the frequency deflecting component as it is to use it for displaying jitter.

CONSTITUTION: A device is formed of a frequency deflection detecting unit 2, to which the serial digital signal S1 is input, and a frequency deflection-cycle deflection converting unit 4. The detecting unit 2 receives the cycle-to-be- measured specifying signal TS to measure each time interval jitter of a waveform part at a different cycle. The detecting unit 2 obtains the frequency to be measured corresponding to the specified cycle to be measured, and detects the frequency deflecting component from the frequency to be measured inside of the input serial digital signal. The converting unit 4 receives the signal having this detected frequency deflecting component, and converts the frequency deflecting component from the frequency to be measured too the cycle deflecting component from a cycle to be measured. This cycle deflecting component is measured as a jitter of the cycle to be measured of the serial digital signal.