1. A method for monitoring a system which includes a plurality of nodes coupled by a network, the method comprising: managing a data structure including a condition object and two or more cause objects, the condition object being linked to the two or more cause objects by considering a topology of the system; updating the condition object on receipt of an event message which notifies that an event corresponding to the condition object has occurred on a node in the system; and calculating matching ratios for causes related to the two or more cause objects based on the data structure including the updated condition object.