A system such as an accounting system providing highly reliable continuous operation services at low cost. The system is configured to transfer data asynchronously between a main system that operates in daytime mode and a subsystem that operates in nighttime mode. A sub-database receives and stores only data required in transactions performed in nighttime mode from the main system and its main database. The subsystem, which is responsible for performing processes in nighttime mode, transfers a log of transactions performed in nighttime mode to the main system through a subsystem queuing box on returning to the daytime mode and data re-processed by a main processor in the main system is stored in the main database based on the transaction log.