With respect to one order information piece in security transaction, processing of reception, transaction condition confirmation, transaction settlement and result transmission is executed serially. A transaction system is configured so as to divide a transaction including a plurality of transaction requests into predetermined units and execute resultant processing units in parallel. When such division is conducted, the sequential property is ensured. For example, the progress situation of processing is stored in a database every processing and every step. Issues, terminals, markets and servers are included as predetermined units.