1. A secret key generating method for generating a secret key of an entity in each of a plurality of key generating agencies, comprising the steps of: obtaining each divided identification information by dividing identification information of the entity into a plurality of blocks; extracting a part of the components of a symmetric matrix of a secret for each key generating agency in accordance with each divided identification information; and generating a secret key of the entity by synthesizing the extracted components with a random number particular to the entity.