1. A computer readable storage medium including instructions that, when executed by an intrusion detection system, cause the intrusion detection system to perform an intrusion rule storing method for storing a new intrusion rule in storage having already stored intrusion rules, the method comprising: generating a plurality of division combinations, with each of the division combinations capable of dividing the new intrusion rule into a plurality of partial intrusion rules; with respect to each division combination, calculating a hash value of each of partial intrusion rules that are included in the division combination, and calculating a frequency of collisions between the hash values of the division combination and the hash values of the intrusion rules already stored in the intrusion detection system; dividing the new intrusion rule according to a division combination having the lowest calculated frequency of hash value collisions; and storing the divided new intrusion rule in a corresponding position of the intrusion detection system.