A monitoring system for monitoring a service execution infrastructure for providing a service to client computers via a network manages baselines of monitoring values of components per load of the service provided by the infrastructure, and uses the baselines depending on a current service load. When detecting an abnormality of a service monitoring value or component monitoring value by use of the baselines, the monitoring system compares events up to predetermined minutes ago from now with events in the baseline time zone thereby to specify a differential event (or non-normal recent event).