PURPOSE: To decide a compressed part and an expanded part, respectively of an external storage device by inputting test data to an external storage means and comparing compressed data with an output of a read-out compressing means.

CONSTITUTION: When test data is generated from a test data generating means 1, this test data is inputted to an external storage means 4 and compressed and stored, and the stored compressed data is read out and inputted to a compressed data comparing means 3. Also, the test data is inputted to a compressing means 2, as well, compressed and inputted to the compressed data comparing means 3. The compressed data comparing means 3 compares the compressed data inputted from the external storage means 4, and the compressed data inputted from the compressing means 2, and when they are same each other, it is decided that a data compressing function of the external storage means 4 is normal. Moreover, an expanding means is provided and data obtained by expanding the data compressed by the external storage means 4, and data expanded by the external storage means 4 itself are compared by an expanded data comparing means.