In a communication system in which a plurality of processing apparatuses are communicably connected to one another over a network, a processing apparatuses has an update request issuing unit issuing an update request containing address information to be updated in order to update an address book and an address book updating unit updating the address book on the basis of a response result to the update request, a determining unit determines whether or not the address information contained in the update request is valid when receiving the update request from the processing apparatus, and a response result transmitting unit transmits a result of determination made by the determining unit to the processing apparatus as the response result to the update request. The address book of the processing apparatus retaining the address information about other processing apparatuses can be updated to the latest state, with certainty.