A system has a plurality of information processing apparatuses, each comprising: a storage device that stores, for each occurrence pattern of events to be detected, a plurality of tasks for respectively determining whether a plurality of conditions are fulfilled; a process determination section that, in response to occurrence of an event, reads the plurality of tasks corresponding to the occurrence pattern including the event from the storage device, and determines which information processing apparatus is assigned to process each read task; a process execution section that processes a task assigned to be processed by the information processing apparatus, and instructs another information processing apparatus to process another task assigned to be processed by the other information processing apparatus; and a detection section that determines that the event has occurred if process results of the processed task and the instructed task fulfill the plurality of conditions.