PURPOSE: To prevent the managing mistake of the maintenance level and to improve the reliability, by storing the correction and the maintenance level up to the newest maintenance level to plural stored maintenance levels, and changing the storage level into the newest level at the maintenance.

CONSTITUTION: A data processor 1 is formed with a maintenance level storage section 3, a program storage section 7, and a program maintenance processing section 4, and is connected to a magnetic disc 2, which stores correction command sections 5-1W5-n having respectively the correcting content AMD1Wn and present maintenance levels L1WLn and the corresponding newest maintenance level LL in a storage medium 5.In starting the maintenance processing section 4, a maintenance level MLC is read out from the maintenance level storage section 3, the magnetic disc 2 is accessed and the level is compared with the present maintenance levels L1WLn at each correction, and when a coincident level exists, the program of the storage section 7 is corrected according to the correcting content AMD corresponding thereto and the level of the storage section 3 is changed into the newest maintenance level LL of the storage medium 5, allowing to prevent the managing mistake.