PURPOSE: To efficiently record operation data and improve the reliability of the recorded data by providing a means for compression recording and using a non-volatile memory for a recording medium.

CONSTITUTION: Since a frequency processing operation part 18 of a control unit 14 is provided with a frequency processing function of converting operation data to frequency distribution data, the operation data with a large fluctuation width can be compressed and recorded in a small capacity memory. Also, the average value per a constant operating period of the operation data is calculated. Since it is also provided with an averaging processing function, the operation data can be compressed and recorded in a small capacity memory, thus diagnosing the operation state over a long time. Then. compression data which are processed by the operation part 18 can be recorded in a non-volatile memory provided semi-permanently within the unit 14 and can be read from a floppy disk, etc., as needed, thus drastically improving the reliability of record data.