System (software) development is performed based on a state transition diagram. The workflow of a business process is partitioned for each event therein and assigns process codes to the respective process results. A process code is an identification code to indicate a certain state of a process. For example, a process code AA is assigned to the waiting state before completion of a process A, and a process code AB is assigned to the completion state thereof, wherein the process code AB is set as a condition to start a process B. The process codes that are set based on the state transition diagram are managed in an external database (DB) so that flexible modification of the state transition configuration and the state transition order is possible. This provides an entirely new method of system design that facilitates business process reengineering.