To provide a storage device for efficiently storing data, over a maximally long period.

The device includes a plurality of stages of ring buffers for storing a prescribed number of recording data respectively, and a plurality of mean value calculation means for calculating respectively a mean value of a plurality of recording data stored in the ring buffer on each stage, and each mean value calculation means calculates a mean value, each time a prescribed number of recording data are written in each ring buffer and writes the calculated mean value in the next-stage ring buffer.