A modulation error is detected every symbol data to generate a trigger signal. The present invention focuses that there are limited patterns of shifts from one symbol data to the next one of the digital modulation signal. Measured values of amplitude, phase and/or frequency of symbol data are latched and then values at the next symbol timing are predicted from the latched measured values using said feature. The predicted and measured values are compared at the following symbol timing. If the difference (error) is over an acceptable range, a trigger signal is provided which allows acquiring a modulation error by symbol data.