PURPOSE: To improve the reliability of the system by comparing data on a local area network with data storing the operation result of a computer to a database so as to grasp the operating state of the computer.

CONSTITUTION: A data read section 23 reads data from a storage area 21 of an analyzer section 19 and outputs the data to a comparison error analysis section 35. Moreover, a data read section 31 reads data from a storage area 29 of an analyzer section 27 and outputs the data to the analysis section 35. The analysis section 35 retrieves a database 37 to check whether or not the operation of a computer 15 with respect to a LAN 11 is normal and discriminates the operation to be normal when the data are in existence in the database 37 and discriminates the possibility of protocol violation when no data are in existence. Thus, the data sent/received to/from the LAN are analyzed and the data from the computer are analyzed, then the operating state of the computer is grasped and the reliability of the system is enhanced.