To provide a data averaging system capable of accurately averaging digital data in a short time, and to provide a disk device therefor.

This system comprises a low-pass filter processing part for processing digital data DCV from an A/D converter part by low-pass filtering, and an averaging processing part for averaging digital data DLP from the low-pass filter processing part. When the signal waveform of the input digital data DCV to the low-pass filter processing part is changed stepwise (adjustment parameter for deciding the voltage level of AS is changed, when DCV is changed over) or before the averaging processing is started, the contents of the arithmetic processing in the low-pass filter processing part are changed so that the DCV is outputted as it is as DLP (so that the signal waveform of DLP is changed stepwise). When a signal for detecting the abnormality of the AS becomes active, the low-pass filter processing and averaging processing are halted, and the averaging processing is started from the sampling timing immediately after the detection signal becomes inactive.