PURPOSE: To avoid complication of a knowledge expression and to avoid complication of the knowledge processing even in the case that various kinds of conditions exist as conditions for cause discrimination in a causal relation network.

CONSTITUTION: Events as searching objects are linked by links of causal relations L1 to L6 and are expressed as a tree structure by a knowledge, and conditions for retrieval of an event as the cause of a prescribed event from the tree structure are expressed as condition events, and these condition events are related to events themselves as retrieval objects by links, and all conditions are expressed by the same knowledge expression. Since all conditions are indicated by the same knowledge expression in this knowledge base, complication of the knowledge expression is avoided, and the systematical processing is possible at the time of retrieval.