Abstract: A nuclear fusion reactor (2) or nuclear fission reactor (22) is used as a heat source. A heat exchanger (11 or 37) that contains water to be heated (15) is used for water vapor generation. A circulating pipe (10 or 26) through which a fluid for cooling the nuclear fusion reactor or nuclear fission reactor or for conducting heat exchange circulates is disposed so as to extend in the heat exchanger and be in contact with the water to be heated. Water vapor is thus generated. This water vapor is jetted toward the sky at a state of collimation through a vapor discharge pipe (12 or 36). A cloud for blocking sunlight is formed in the sky from the water vapor jetted to reduce the temperature of the earth surface. This enables a weather modification without discharging any greenhouse gas, e.g., CO2.