To provide an analog signal monitoring system which makes a circuit scale compact, reduces processing time and realizes highly accurate display due to a low resolution ADC(analog/digital converter).

This analog signal monitoring system is provided with an A/D (analog/digital) converting part 2 converting a monitored signal into a digital signal, a memory part 3 holding the display data of a data displaying part 6, a data variation recognizing part 7 that decides whether a digital signal variation is a variation due to the conversion error of the part 2 or an original variation of the monitored signal on the basis of the current display data and the digital signal, and a data selecting means 4 which selects the current display data held by the part 3 in the case of the variation due to the conversion error, also selects a digital signal converted by the part 2 in the case of the original variation of the monitored signal and uses the selected display data or digital signal as updated display data.