A controller stores link information linking a predetermined event to a variable associated with the event. The controller stores first log information in association with time information into a memory in response to occurrence of the predetermined event. The first log information includes information about the event, and a current value of a predetermined variable associated with the event included in the link information. The controller stores second log information in association with time information into a memory in accordance with a predetermined period. The second log information includes information about a value of the predetermined variable. An information processing device obtains the link information from the controller, and displays information indicating the variable associated with an event indicated by the first log information onto the monitor-based on the link information and the second log information.