A communication system for guaranteeing only one each of two processing operations, correlated with each other and executed on different devices. A server has a unit which, on receipt of a first processing request, with identification information, executes this processing only once for one item of the identification, and a unit for transmitting a first processing completion notice to the transmission source of the processing request on completion of the first processing or on re-receipt of the processing request for the first processing with the same identification of the completed first processing. The client has a server, a unit for transmitting the processing request for first processing with identification information, a unit for executing a second processing, previously correlated with the first processing, on receipt of the completion notice, and a unit for re-transmitting a processing request on detection of an error before receipt of a transmitted completion notice.