The invention is related to a system and a method to determine whether a target equipment deviates from a normal state. If it is determined that the target equipment to be diagnosed deviates from the normal state, the degree of deviation of each parameter from the normal state as the reference is calculated as an abnormal contribution ratio. A failure cause is estimated from a similarity ratio between the calculated abnormal contribution ratio and the abnormal contribution ratio of each of the failure causes collected in the past and including failure phenomena and failure parts.