An operation monitoring apparatus for estimating a cause by which a first and second observation data observed by a monitored apparatus are obtained, the operation monitoring apparatus including: an observation event receiving process unit which receives the first and second observation data transmitted from the monitored apparatus; an observation event extracting unit which extracts the first observation data, out of the first and second observation data, based on a probability that the cause occurs with respect to the first and second observation data; a cause event determining process unit which estimates the cause based on the extracted first observation data; and a cause notifying process unit which outputs data indicating the estimated cause.