There is provided a session sharing system for sharing sessions in a system in which one or more application servers cooperate for plural applications including: a first function processor; a second function processor; and a session information cooperation unit for converting session information of a session established with an application server, which is acquired from the first function processor or the second function processor, into a predetermined format and giving the converted session information to the second function processor or the first function processor, wherein the second function processor or the first function processor transmits, to an application server, a request message including the converted session information received from the session information cooperation unit, and the application server associates the converted session information included in the request message with session information that is generated based on a current communication processing protocol.