PURPOSE: To clearly hold crude data without spoiling their feature by only dividing the data into effective and noneffective data by performing data compression after the crude data are transformed into compressing range data and a matrix and effective data arrays.

CONSTITUTION: An arithmetic device is provided with a microprocessor 1 having an arithmetic operation function and storage device 2 for storing processed compressed data and the storage device 2 has a header storing area 2a, matrix data storing area 2b, and effective data storing area 2c. Data compression is performed by transforming crude data into compressing range data and a matrix and effective data arrays. Therefore, the crude data can be held clearly without spoiling their feature by only dividing the crude data into effective and noneffective data.