1. An intrusion detection rule simplification apparatus comprising: a rule definition unit defining predetermined fields out of fields of a packet as test items and including at least one intrusion detection rule for defining test orders and conditions of the defined test items; a group rule generation unit rearranging the test conditions of the intrusion detection rules based on the test items, grouping the same test conditions for the same test items, and generating a connection structure of the test conditions so that the test items and orders of the intrusion detection rules are satisfied; and a common rule generation unit generating a common rule consisting of test conditions having first test orders in the generated connection structure.