To reduce an economical risk to be imposed on a dispersed power source user by calculating the gain and loss of the user who purchases a dispersed power source with precision.

Operation state data to be calculated by the operation measuring instrument of the dispersed power source and power usage state data to be calculated by the power measuring instrument of a power user are recorded in a monitoring database. The gain and loss of the dispersed power source user and the power user is calculated based on the operation state data and the power usage state data retrieved from the monitoring database. Information concerning the financial transaction between the dispersed power source user and the power user which is calculated based on the gain and loss of the dispersed power source user and the power user is transmitted to the server of a financial organ. Accordingly, the gain and loss of the user who purchases the dispersed power source is calculated with precision, so as to reduce the economical risk to be imposed on the dispersed power source user.