1. A fault cause estimating system comprising: an instance definition database configured to store an instance definition which defines a correspondence relationship between a plurality of devices as managed objects and classes of the plurality of devices; a fault occurrence location class definition database configured to store a fault occurrence location class definition which defines a correspondence relationship between a plurality of faults possibly occurring in the plurality of devices and classes of the plurality of devices; an event sequence database configured to store an event sequence for fault cause estimation in which a plurality of events having occurred in the plurality of devices under monitoring are arranged in order of time of occurrence; a first storage section configured to store a fault derivation model which shows a fault transition probability concerned with combinations of the classes of the plurality of devices and faults at the classes of the plurality of devices; a second storage section configured to store an event derivation model which shows an event transition probability concerned with combinations of faults at the plurality of devices and events derived from the faults at the plurality of devices; a third storage section configured to store an instance derivation model which shows an occurrence device transition probability concerned with an occurrence device at which a fault occurs among the plurality of devices; an extended Viterbi calculating section configured to independently obtain a first transition probability between a plurality of faults at the classes of the plurality of devices and a second transition probability between the plurality of devices, based on: the instance definition, the fault occurrence location class definition, the fault derivation model, the event derivation model, the instance derivation model and the event sequence for fault cause estimation, and estimates a fault state transition sequence and a fault state transition probability which show a fault transition sequence and a fault transition probability which correspond to the event sequence for fault cause estimation, by multiplying the first transition probability and the second transition probability; and a filtering module configured to estimate a fault which is a root cause of an event occurring at a managed object, based on the fault state transition sequence and fault state transition probability.