To properly decide a discount rate when setting the amount of money to be paid by a customer accompanied with the operation of gas consumption equipment having a power generating function as the amount of money which is smaller than a charge of power to be purchased from the power company.

The history of power consumption by a customer stored in a result storing means 11 is used, and gas consumption accompanied with power generation by gas consumption equipment is predicted by a gas consumption predicting means 13. The fluctuation of the unit price of gas and power in the lease period of the gas consumption equipment is predicted by a charge fluctuation predicting means 14, and the fluctuation of the gas charge to be paid by a customer is calculated by the gas charge calculating means 16 from the predicted gas consumption and the predicted gas price unit. The calculated gas charge, lease charge and the payment sum of each customer in every unit period including a risk fund reserved for absorbing the prediction error of the unit price of power is calculated by a payment sum calculating means 17. According as a difference between the power charge to be calculated from the unit price of a power predicted by the charge fluctuation predicting means 14 and the payment sum calculated by the payment sum calculating means 17 becomes large, large discount rate is selected.

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