PURPOSE: To attain the reduction of storage capacity and the promotion of the efficiency of analyzing work by sampling automatically only communication data in a period required for analyzing the cause, etc., of communication abnormality.

CONSTITUTION: When a data sampling job is selected on a save mode after synchronization in a master machine 1, a CPU 10 collates the communication data transmitted/received between salve machines 2a, 3b with synchronous data set beforehand, and when the coincidence of both data is confirmed, it samples the succeeding communication data by storing it successively in a RAM 12. Besides, when the data sampling job is selected in the save mode before synchronization, it samples the succeeding communication data by storing it successively in the RAM, and simultaneously, each time the communication data is sampled, it collates the communication data with the synchronous data set beforehand, and when the coincidence of both data is confirmed, it finishes the sampling work of the communication data.