In a content retrieval system where an update frequency of content is extremely high, based on a content reference request from a client which generates the content reference request including unique information and an attribute of a desired content, a cache server checks a presence/absence of a cache of the content, caches the content from a content server to transmit the content to the client in the absence of the cache of the content, or transmits the content held to the client in the presence of the cache of the content. Then, the cache server performs synchronization mirroring with the content server based on the attribute.