PURPOSE: To prevent important fault log information from being missed by writing a new fault log information on the old one in a file.

CONSTITUTION: A log information collecting method of an information processing system is provided with a main device 100 which includes a central processing unit and a maintenance diagnostic device 200 which consists of 8 log collecting means 210 which is connected with the device 100 through a diagnostic interface 150 and collects the internal state of the device 100 as a log information at the time of trouble of the device 100, and a storing means 240 which stores the log information collected by the log collecting means 210. In addition, a detecting means 220 which detects that the number of log informations stored in the storing means 240 has reached a certain number, a test diagnosis execution detecting means 250 which detects whether a test examination for maintenance is under execution or not and a suppressing means 230 which suppresses log registration to a certain number while a test diagnosis is under execution when detecting is performed, otherwise that suppression is not executed, are provided.