preferred embodiments fig1 shows the relationship between the total flow rate and the time in the case where the wellhead valve of a geothermal well is gradually throttled in fig1 geothermal wells no 1 and 2 are connected with a geothermal reservoir and the total quantity i of the flow rate produced from the geothermal well no 1 and the flow rate produced from the geothermal well no 2 is produced assume that the geothermal reservoir has a predetermined volume and geothermal fluid i o i i always flows to the geothermal reservoir when the wellhead valve of the geothermal well no 1 is now gradually throttled the internal pressure in the geothermal reservoir increases by geothermal fluid always flowing into the reservoir and the total flow rate of the fluid hereinafter referred to as 34 stopped flow rate 34 stored in the geothermal reservoir increases as time is elapsed as shown by a solid line b in fig1 when the valve is completely closed it is considered that the stopped flow rate does not vary but remains constant as the time is elapsed when the wellhead valve of the geothermal well no 2 is then gradually throttled it is considered that the stopped flow rate again increases as the time is elapsed however when the wellhead valve of the geothermal well no 1 is gradually throttled the stopped flow rate in fact varies as shown by a solid line a in fig1 when the valve is completely closed the flow rate gradually decreases from the.